Tank

A French Renault FT-17 Tank, the first tank to have the rough layout that would be the norm for tanks (Crew in the front, top mounted 360 degree turret for main gun, engine in back)

A tank is a tracked, armored combat vehicle. The term is often limited to vehicles intended for direct combat, as opposed to e.g. self-propelled artillery (which stay to the rear) or armored personnel carriers and infantry fighting vehicles, which are on the front line but are primarily tasked with carrying soldiers as opposed to fighting directly, and may not necessarily be tracked. Their invention revolutionized warfare in the 20th century, and any wargame set in or after that time period (or in alternate universes with similar or more advanced technology levels) will have plenty of attention devoted to them -- or to whatever made them obsolete, as in e.g. BattleTech.

The idea of an armored fighting vehicle dates back at least to Leonardo da Vinci and H. G. Wells, but the modern tank was proposed shortly before World War I, and was then spurred to production by the war itself. When the war on the Western Front got bogged down in trenches, the British Royal Navy, who had already had some success with mobile armoured car groups, had the idea to use tracked, armored vehicles with guns to break the stalemate. The name "tank" became attached to the vehicle as a codename to disguise the purpose of the large metal bodies being built. After the first tanks rolled onto the battlefield, other countries called them "battle wagons", "armors", "assault vehicles", and other more descriptive names, but the Anglosphere was stuck with calling them "tanks". (Interestingly, the original British Tank, which looked like a tractor with a metal box on top of it was called "Little Willy".)

Common Features of the Tank

Tanks were built with pretty much any set of features you could imagine, but over time, the militaries of the world settled on several key features:

1. A large-caliber primary cannon for destroying enemy vehicles and defenses. This is partially why the Navy was the first branch of the British military to design and produce tanks during WWI, but mostly because the early designs demonstrated for the British army were unimpressive at best while First Lord of the Admiralty Winston Churchill saw their potential. Tanks armed with missiles instead of a main gun have been tried, but they have yet to take off.
2. A turret to house the tank gun, to allow the tank to shoot at targets without having to pivot the entire vehicle. The French had this one figured out by 1918, and some tanks developed during the interwar period actually had more than one (though this proved impractical). Strictly speaking this one is not essential as the German Stug proves, and indeed the lack of a turret does have some advantages thanks to how it lowers the profile, but the advantages of a turret are strong enough to render the benifits offered moot. A turret-less tank is only really useful if you don't have the money to make a turreted tank, or you don't have the technology to mount a gun as big as you want in the turret.
3. A sloped, heavily-armored front face to absorb attacks, including those from enemy tanks. The reliance on thick front armor led to the development of anti-tank weapons like missiles and mines specialized to attack more vulnerable rear, underside, and top armor. All that said, sometimes designers choose not to armor their tanks. If the thickest armor you can put on a tank is going to get penetrated anyway then your best bet is not to armor it at all and focus on speed to make sure you don't get hit to begin with. During the Cold War, the French made heavy use of this designs, especially during the period when new HEAT shells had made steel armour less useful, but newer and stronger composite armour hadn't been invented yet.
4. Tracks with a profile as low as possible while meeting all-terrain mobility needs. A stopped tank is a dead tank, and running the tracks over the top of the body is begging for a mobility kill, no matter how cool the British Mark I looked. (Though to be fair to the Mark I, it needed its high tracks to cross trenches, and since it came first, there weren't really any weapons that could take advantage of its exposed tracks at the time.)
5. A radio! It can not be overstated how important a radio became to tanks. In both the battle for France and the early Operation Barbarossa the German tanks were under gunned and under armored compared to their opponents, but thanks to their radios, they were able to outmaneuver the enemy and take them apart. Radios also became important to tanks because, well, tanks are loud, and it's the only way for the crew to talk to each other without going hoarse yelling at each other.

Basically, tanks boil down to three features: mobility, defense, and firepower. You can generally speaking pick two at the cost of the third. The heavier your guns and armour, the slower the tank will be, for instance, while a vehicle made for mobility has to sacrifice either protection or the size of its guns.

Useful Accessories for Tanks

Particularly true of modern tanks, while the above features are considered absolute essentials, most tanks will come with a number of additional features to improve survivability and combat effectiveness.

Offense

• Machine Guns - These are useful for dealing with infantry in situations where a cannon is overkill or can't be brought to bear quickly enough. While WWI tanks liked to stick sponsons on all sides, the popularity of the turret made it much more practical to limit the number of secondary weapons to stick on. Typically, one is mounted on the top of the turret to be manned independently, or else added as a coaxial weapon to the main gun.
• Ammunition types - While all cannon shells explode, the way they explode affects their performance against specific targets. For example: an anti-tank round focuses its explosion onto a single point, making it deadly against large armored targets as it can force its way through their armor, but not so much against infantry, as unless you score a close or direct hit with them; its not going to reliably kill them. Conversely, anti-infantry rounds send shrapnel flying everywhere upon detonation to wound clusters of infantry and damage light vehicles around the explosion, but because they do not carry a big-enough payload and the force of the explosion just scatters everywhere; they cannot reliably threaten heavy-armored tanks. Thus, a tank needs different types of ammunition to engage different targets efficiently, otherwise its just wasting ammunition.
• Auxiliary Launchers - Even though you already have a cannon, having one or two of these on hand can still give you a leg up for any lopsided tank battles you may encounter. While most battle tanks eschew guided missiles due to their main gun being enough, lighter tanks typically use wire-guided missiles or dumb-fire rockets to give them an extra anti-armor punch when needed. Additionally, lighter tanks benefit from guided AT missiles as it allows them to reliably threaten heavier tanks, which could shrug off the shots from their low-caliber main gun.

Defense

• Smoke Launchers - The little pipes you see on the turrets and hull of the tank are smoke launchers, which fire a single salvo of smoke grenades upon activation. You may find the idea of trying to hide a tank ridiculous, but a good tank commander will know how to use smoke to mask their movements in case they need to make a hasty retreat. However, they're also really, really good at fouling up enemy laser guidance systems in an emergency situation or simply confusing AT infantry trying to get a bead on them.
• Reactive Armor - Because modern MBTs do not have the same armor plating of heavy tanks, a way to improve their survivability came in the form of reactive armor. Reactive armor are boxes with a metal plate and a small explosive charge behind them (although modern variants have non-explosive versions for the safety of the friendly infantry around the tank). When an AT-round attempts to penetrate the reactive armor; the charge inside ignites; causing the metal plate to fly out and intercept the AT round before it does any actual damage. Each plate is only a 1-time use thing, but very invaluable for keeping your heavy hitters mobile yet durable.
• Slat Armor - For shape-charge rounds, something as simple as a metal cage surrounding you can prevent the shape charge from going off by crushing it.
• Improvised Armor - Just like its name says; its tank crewmen trying to bulk up their tank using unconventional things. Depending on their luck and resourcefulness, these can take the form of salvaged armor plates from other tanks, or nothing more than materials they find in the field, like sandbags or wooden logs. The latter two were popular in WW2, among other things. Improvised armor can protect the crew from some light anti-armor weapons to some extent, but does not fare well against heavy launchers and tanks with large main guns.
• Active Protection System - An APS is a device that shoots down incoming anti-armor projectiles using a weapon. It works by having an active radar detect incoming AT rounds at the tank and then the weapon its using (like a rocket launcher), will then fire at it to destroy it mid-flight. A great way to avoid damage altogether, but it is limited by its ammunition capacity and that it starts becoming less effective when faced with multiple AT rounds coming it's way.

Support

• Autoloaders - The main advantage to automating the loading process is that you could afford to have one less crew member, thus allocating better use of space and reducing crew complexity. That said, it does make your tank heavier, expensive, more maintenance-intensive, and your autoloader goes along with your tank if its destroyed (a human has a chance of ejecting from the tank if destroyed and return back to his lines to man another tank in no time, the auto-loader has to be salvaged by an engineering team, just to assess if its still usable). Also, a human loader can do things besides loading, as the need arises and until recently could load faster. Not to mention autoloaders are mechanically tricky, some Russian Autoloaders had the bad habit of every so often loading the gunner's arm.
• Sensor arrays - Tanks have notoriously bad situational awareness, so your only options are to stick your head out and expose yourself to gunfire, or get yourself some sensors and cameras to do the looking for you.
• Remote Weapon Systems - The pintle-mounted gun is great for clearing out and suppressing infantry because it can rotate 360 degrees and is at the top of the tank, so it has a bird's-eye view of the area. Unfortunately, popping your head out to shoot at people makes you a prime target for snipers. A solution for this is by having the pintle gun be virtually controlled from the inside by a remote weapon system, so the gunner can still shoot at targets without threat of catching a bullet in the face the moment they peek out of the hatch. Granted the gun can still be shot, but its a lot easier to replace a machine gun than a trained crewman.
• Extra Tracks - A sitting tank is a dead tank, and the best way to stall a tank is by targeting its vulnerable tracks to keep it from being mobile. Having extra links of tank tracks stored on the tank can allow your crewmen to make quick repairs, in the event that its immobilized due to this.

Crewmen

A tank isn't something that one person can fully operate alone (atleast for today's standards), its a large, complex machine that requires multiple people with specific tasks to keep it working. Never discount the importance of a well-trained crew, as they can be every bit as important as the selection of equipment. While technically feasible to operate a tank with only two people (a driver and a gunner, as was the case with the Renault FT), several impracticalities often necessitate having more people per tank to divide the workload. Most tanks today have a crew of three or four, with some crewman having multiple duties to keep things as streamlined as possible.

• Commander - The commander is the one who issues orders to all crewmen. Their main responsibilities for the tank are navigating for the driver, spotting targets for the gunner, and coordinating everyone to work as one well-oiled deathmachine. They're also the one who typically mans the hatch-mounted machine gun. In modern times, they are also responsible for the radio.
• Driver - Maneuvers the tank. Because the driver is typically near the bottom of the tank and so has their vision restricted; they have to rely on the commander for precise maneuvers when the tank is traveling at full speed. That said, it is still possible for a driver to maneuver the tank on his own. In modern times, they also serves as the crew's mechanic.
• Gunner - Operates the tank's main cannon(s) and coaxial machine gun. He's responsible for aiming the guns where they need to shoot and firing when appropriate. They can also double as a loader if one's not available. Because the gunsight is quite narrow, they can only make fine adjustments on their own and need the Commander to tell them which direction to point in the first place. A tank typically only needs 1 gunner, but older models that have more than 1 main cannon (like the WW1 British Mark tanks) required a gunner for each gun.
• Loader- Assists the gunner by loading the appropriate ammunition into the main gun. Loaders are typically not present in modern tanks anymore as that position is automated these days, but older tanks needed them to perform efficiently.
• Mechanic - Responsible for fixing up the tank when it breaks down. In later years to ease space concerns; the driver typically doubles as the crew's mechanic.
• Radio Operator - Operates the radio and relays any orders and communications with friendly forces to the commander. Due to advancements in radio technology; radio operators are no longer needed in modern tanks as the commander can do that on their own these days.

Types of Tanks

Whether it's real or fantasy, tanks are classified from their weight and/or armament profile. Here are the common ones:

Not Actually Tanks

Despite having treads and a gun, the following vehicles are not considered tanks. The difference is that tanks are designed for frontline Combat", while other vehicles with treads are designed to carry and support infantry (APC/IFV), bombard enemy positions with heavy artillery (SPGs), or act as general support weapon systems. Many of these vehicles are light enough to be deployed by aircraft, giving them an edge over tanks in response time to emerging threats.

• Armored Personnel Carrier - APCs are light vehicles designed to carry infantry and not much else. They're usually given a heavy machine gun to support the infantry they're carrying into battle and to defend itself, and not much else. They're designed to protect against small arms fire, not tank shells. Unlike IFVs, APCs are not expected to fight on the front due to their lackluster protection and armaments. However, they're sometimes also amphibious, something that the vast majority of tanks are not, allowing for both seafront assaults and quick getaways down waterways.

• Self-propelled gun (SPG) - Vehicles armed with artillery weapons designed to bomb the enemy back into the stone age, ranging from howitzers, mortars, or missile systems. Typically built similarly to tanks, but sacrifice armor for their heavy guns since in normal circumstances they should be too far away to get shot at directly. Not to mention that artillery pieces have a minimum range where they can drop their payload; thus, the tank needs to put some distance between them and their target so that they can be in effective range. The advantage to having such artillery on an actual vehicle rather than being stationary, is that counter-battery fire can threaten static guns, while mobile guns can safely get out of the danger zone once they've delivered their payload. Self-propelled guns typically carry a 150+mm Howitzer, much larger than what any proper tank would carry. While mobile rocket platforms such as the BM-21 or MLRS are more popular than Self-propelled guns, and are capable of absolutely soaking an area in rocket slam, the SPG has the advantage of being able to sustain fire fire longer periods of time.

• Self-propelled anti-aircraft gun (SPAAG) - Tank-like vehicles armed with weaponry designed to shoot aircraft out of the sky to provide mobile anti-air cover. There are only three real ways to shoot a very fast moving aircraft out of the sky. First you can use as many rapid fire guns as you can to fill the air with as many bullets as you can and hope for one hit. Second, you can fire one big shell up into the air and at a certain height have it explode spraying shrapnel around it self to score the one hit you need, this are known as Anti Air Artillery, and are known in the English world by the name the Germans used during World War 2, flak. Both have been superseded by AA missiles which can track a target and put that shrapnel warhead closer to the target than just guess work and a slide rule can.

• Infantry Fighting Vehicle - Known as IFVs, these tanks are capable of transporting infantry forces, while being armored and armed enough to be of support to the field, unlike light tanks. However, unlike true tanks, IFVs can't be expected to stand up to enemy armor. Modern IFV's can have anti tank missiles, but with their tin can armor, going toe to toe with a main battle tank is suicide and so it supports regular tanks or takes on enemy armor in emergencies. An IFV's goal is to deliver its infantry and hang around to use its 20mm+ autocannon to support the troops it drops off. Most often used for airborne or scouting missions.

• Tank Destroyer - Tank destroyers are specialist tanks designed for one thing in mind: knocking out other tanks and not much else. They tend to prioritize a really, really big gun, often at the cost of armour, and mobility is thus much more important for them. Some are turreted, some aren't, so the definition can get a little vague. What makes them not tanks is a matter of technicality. Tanks are general purpose while Tank destroyers (When they existed) are for only one thing, destroying other tanks. After World War 2 we figured out that since tanks fought other tanks so often anyway tank destroyers don't really make sense so we just put tank destroyer guns on regular tanks. A handful of Tank Destroyers still exist, though most of them are really just an IFV chassis with a tank gun mounted to the turret; they tend to be popular with airborne troops in need of anti-armor capability since a proper tank tends to be too heavy to airdrop.

• Assault guns, Similar to tank destroyers, assault guns differ in one important way: instead of an anti-tank gun, they're armed with a anti-building weapon, frequently a howitzer. These tended to be fairly big and fairly heavy compared to SPGs, because they're made to get in close to heavy fortifications. After World War 2 assault guns became light air-dropped weapons to support airborne troops if they encountered hard targets.

Proper tanks

• Tankettes - Less of a tank and more of an armored clown car with guns; these were in vogue for a while in the 1930s. They're essentially a one or two-person tank, armed with machine guns, flame throwers, or anti-tank rifles and not designed to move much faster than the infantry around them. They're generally made to act as mobile infantry support or anti-tank weapons. Needless to say, this idea didn't stick because even high-caliber machine guns (which WW2 was rife with) could penetrate the armor of the tank, making it useless in straight-up combat. Tankettes however, were still fielded in limited quantities after WW2 due to their light weight that allowed them to be safely air-dropped, mainly for non-front line use like tank destroyers, AA guns, and recon vehicles. The only tankette still in use is the German Sedan-sized Wiesel, an airdropped scout vehicle.

• Light Tank - These are lightly armored tanks that sacrifice armor and firepower for maneuverability. They are not meant to be front-line combat tanks, as their armaments are usually too underpowered to go against heavier vehicles, rather they're usually relegated to reconnaissance duties and infantry support. Light tanks would carry guns that ranged from 37mm-45mm, though some early German Panzers were only equipped with machineguns.

• Heavy tank - The big boys, armed with the biggest guns and the thickest armor. Heavy tanks are what you send to crack an enemy defensive line as they slowly rumble forward, guns blazing, destroying anything in sight. . . Except eventually Medium tanks, which split the difference between light and heavy tanks having more firepower than the former and more mobility then the latter, are just more cost effective and Heavy tanks are not too much better than improved mediums, which evolved into the main battle tank. Heavy tanks typically carried 88mm-120mm cannons.

• Main Battle Tank/Medium tank - Medium tanks, which were generally made to carry guns close to a heavy with mobility not far off a light, evolved into main battle tanks. These would become the primary tank for modern nations by combining high speed, adequate armor and most of all a powerful gun. MBT's are not as heavy as we could theoretically make a tank (although modern advances like reactive armor plates, have allowed them to still be as survivable as true heavies) but their speed makes up for it and they act as the spearhead of an assault force designed to create and exploit a gap in enemy defenses to allow massed mechanized forces to rush though the gap. WWII era medium tanks carried 75mm-90mm cannons, though the modern generation of tanks generally have a minimum cannon size of 120 mm or so.

• Infantry/Cavalry tank - A British and French design doctrine, the theory for the design goes like this. Infantry tanks support infantry, (hence the name) and therefore they don't need to go fast and can carry heavy armor while their guns did not have to be terribly strong to support the infantry. However they were too slow to use the line breaks they created (a problem in World War 1) hence the need for the Calvary tank. Cavalry or cruiser tanks were much, much lighter and were designed to move fast and rush though a gap the Infantry tanks made but could not use and create havoc behind enemy lines cutting communication, destroying supplies, etc. The idea was sound, however technology advanced so that Cruiser tanks could have the armor of a infantry tank without the slowness, and infantry tanks could have the speed of the Cruiser's meaning the distinction became meaningless. Before World War II, Russia had a similar idea for three different types of tanks, a breakthrough tank acting as an infantry tank, one tactical breakthrough tank, and a 'fast tank' to exploit gaps. The Cavalry tank role has been pretty much taken over by airdropped armored vehicles such as IFVs or Tank Destroyers.

• Flame tank: a tank with a flamethrower, only used in the 1920s-1950s as they were quickly rendered obsolete as it was more cost-effective and efficient to have standard tanks have a flame thrower as a coaxial gun, though incendiary weapons of various sorts are still used today, mainly in artillery roles.

• Super Heavy Tank - Superheavies were conceived in World War I, essentially using the term "landship" literally. Armed with giant cannon (sometimes multiple ones, and usually reserved for artillery or battleships) and armor plating so heavy, you'd mistake it for a fortress; they were meant to be the ultimate line breakers. While some prototypes were fleshed out, none (except for the French Char 2C, although it arrived too late to be used in WW1 and was obsolete in WW2) were put into service because they were simply too impractical. They were often too heavy to be supported by most roads (and off-road would have been worse), they moved too slow to be a threat, and were a logistical nightmare since their engines guzzled gas like no tomorrow. Overall, commanders found out that it was much better to send out multiple medium/heavy tanks to do the job, than sending a single super heavy. Still, because the idea of a multi-turret warship on treads is universally hella cool, that didn't stop writers from including such weapons in the arsenal of their armies, just to show how powerful they are. Super-Heavy prototypes had cannons that could range between 120mm-280mm, with the Nazis having many of the more ludicrous designs.

Tanks in Warhammer 40,000

Since Games Workshop seems to think that tank development in the Warhammer 40,000 universe stopped somewhere between the World Wars, most of the iconic fighting vehicles of the Imperium are a mish-mash of modern and historical designs. We can quibble that some of those vehicles are not really tanks but armored personnel carriers (e.g. the Land Raider) or other specialized classes of armored fighting vehicles, but GW's design team has a serious problem with looping their tracks all the way up and over the chassis for that 1918 flavor, and not even in the correct direction for what they were designed for. A variety of other vehicles in the 40k universe are referred to as tanks, but these are often hovering vehicles like those used by the Tau or Eldar, and thus technically don't count.

Character Role

In many role-playing games, particularly the online ones, the term "tank" has also arisen to describe a character whose primary purpose is redirect all damage from enemies to himself. This was one of the primary purpose of actual tanks as well; tanks, being as armored and threatening, are supposed to get most of the enemy's attention while the squishier units like infantry and light vehicles move into advantageous positions to deal more damage, without the threat of serious retaliation.

See, many enemies in RPGs have way too much health, deal way too much damage for most classes to withstand, and fights with them are unlikely to be decided in one round unless they're uncharacteristically vulnerable to save-or-die rays (which almost never happens).

Furthermore, many of the classes that are best at dealing damage (assassin and wizard types, for example) often have very little survivability when it comes to being punched in the face, in order to balance out classes (If a class can both tank damage and deal high damage at the same time, they either render other classes redundant or can do neither as well as a dedicated tank or damage dealer).

Thus, demand is created for a character whose job is to redirect enemies' aggression away from the squishy members of the party and towards them instead, usually using their mastery of mind-control, irritating sound effects, imposition of dangerous effects for attacking anyone else, or simply cutting insults and rude gestures which draw attention to themselves. They also tend to have abilities that help them in resisting, mitigating, avoiding, or regenerating from some of the damage they suffer (and on occasion act as a secondary damage dealer). In most cases, tanks are also often reliant on healer classes as well to keep them alive while they do their thing, as enemies that require tanks can usually deplete a good chunk of their health in a few attacks. Making sure that chunk is constantly restored is required to make sure they can keep at it.

Fourth Edition refers to this role as the "defender," while Dawn of War 2 vets will recognize it as the "Tarkus", and later the "Diomedes." While it is most obvious in online video games, the necessity of drawing fire away from squishier party members toward tougher ones who can take a beating exists in a variety of different games, from cooperative card games to MOBAs. The wargame equivalent would be the DISTRACTION CARNIFEX.

Vehicle Warfare
Combat Aircraft - Siege Weapons - Tank - Warship