Crossbow
The crossbow is named for its cross-shaped design, as it is basically a bow on a stick, that uses a mechanism to trigger the release of either an arrow (called a bolt or quarrel) or a small spherical stone or lead bullet (the latter usually being used for hunting). Bolts are arrows with a shorter shaft, and could range from being lighter than an average arrow to several times heavier. It operates on the same principle as the traditional bow in that a tough bowstring is pulled back to store potential energy in the bow, which upon release of the string, transfers it to a projectile.
While historians are unsure who first made the crossbow, the earliest known crossbows were found in China around 500 BCE. During the Warring States period, crossbows became a preferred ranged weapon of Chinese armies and the use of crossbows certainly aided in the rise of the Qin Dynasty and the beginning of Imperial China. The Greeks and Romans experimented with hand-held crossbows, but they never made extensive use of them, although they did make extensive use of ballista (basically a crossbow scaled up to the size of an artillery piece, and often shot stone instead of scaled up arrows). Chinese of these times on the other hand had crossbows of all types and shapes from one-handed repeater crossbows, capable of launching dozens of (fairly weak, but often poisoned) arrows per minute, to absurdly heavy ones, designed to be pulled by legs, rather than arms, and launch arrows the size of small javelin, later with gunpowder-filled bombs on the heading at range, only rivaled by siege engines, though later they passed out in favor of multiple rocket launchers (yes, you read it right). Crossbows began to see widespread use in Europe around 1000 CE, at this time emerged crossbows with steel bow sections, more commonly called Arbalasts.
The difference between a bow and a crossbow is that a bow's string needs to be pulled back and held by the user while aiming, while a crossbow has a mechanism that locks the bowstring in a readied state and only requires the user to operate the trigger to release the bolt upon sighting their target. The trigger mechanism evolved over time, as did aids for drawing the crossbow's string. The weight or size of the crossbow certainly had an effect on the mechanisms involved, as light crossbows could be reset by hand, but heavier version could end up using levers or crank-operated windlasses to pull back and cock the string, and tended to use more robust release mechanisms due to the increased stress involved.
In fantasy settings, technologically advanced races who don't (or in limitation) employ firearms as their go-to weapon typically make heavy use of crossbows. It is also the ranged weapon of choice for more richer and experienced mercenaries.
For some reason in a lot of fantasy and medieval fiction it seems to be the preferred ranged weapon of whoever the bad guys are. Examples include A Song Of Ice and Fire (where it seems to be the favorite weapon of King Joffrey), The Lord of the Rings (Uruk-Hai marksmen use crossbows) and different settings of DnD (Drow poisoned repeater crossbows). This may have something to do with crossbows being less relying on the wielder's strengths, stamina and personal skill, and thus regarded as "unfair" weapon compared to bow, it was at one point banned by the pope for use on Christians, although due to it's usefulness on the battlefield this was largely ignored.
Use in warfare
A major advantage of crossbows over regular bows is that because they're fired using a mechanism instead of depending on human strength to hold the bow in a ready-to-fire position, user fatigue is no longer such a huge factor. Additionally, the heavier crossbows could generate more force than most humans thanks to the pulley systems used to cock the string, thus heavier bolts could be thrown, resulting in greater penetration of the target. Possibly its biggest advantage is that it was easier to train the use of a crossbow than bows since the weapon's operation is much less taxing.
Of course, the main drawback with crossbows is that they require a wider range of resources and skills to produce due to the mechanisms involved. Crossbows also generally have a lower rate of fire than bows. At best, a crossbowman can get off about two shots a minute. More powerful arbalast crossbows that used windlasses took longer, but could store more energy than a human could physically pull back. For these reasons Crossbows excel in a siege situation where the ranges are long and you can duck into cover easily while your reloading. And since sieges tended to be drawn-out affairs anyway, reload time wasn't as necessary.
It should be noted that despite having much greater draw weights than contemporary bows (above 500 lbs in some cases), crossbows were not proportionally more powerful due to their very short draw length, translating into a much briefer energy transfer. More modern crossbows sometimes address this by using recurve bows, or even by using a bullpup configuration by turning the bow backwards and then pulling the string past the bow.
The fact that even relatively poorly trained men armed with crossbows could royally murder fully armored knights, made the crossbow one of the most hated pre-firearm weapons in the Europe, even more than the infamous flamberger. At some point it even came to the pope banning crossbows as unholy weapon not to be used on fellow Christians, but even then they remain to be popular amongst mercenaries, rich lords and protestants. Proud knights could not accept the fact, that some dirty peasants could kill them only because their lord could afford few dozens of crossbows.
Crossbows are still occasionally used for military purposes; while a crossbow's bolts lack the stopping power of modern firearms, the bolts it fires are quieter than any "silenced" firearm and it can also be used for niche purposes such as launching grappling hooks and it's lower projectile speed means it's much less likely to set off any worn explosives. It should be noted, that using crossbow for silent takedowns is generally a bad idea, as while if fires silently, the victim usually don't die instantly, unless you shoot in the eye or throat, which means if have from few seconds to few minutes to scream until it bleeds to death, thus setting an alarm. Although you could use the same solution that the Indian Navy used for this problem and use Cyanide tipped bolts
See Also: Crossbows are Underpowered in d20
Loading Mechanisms
Because later crossbows were often too tough to simply pull back unaided, a number of devices were invented to allow the wielder to pull back the string. The device used usually depended upon the weight of the bow, as heavier bows would require more advanced devices that required more time to pull back.
- Goat's Foot Lever: This was a fairly simple detachable lever that gave the user more leverage when pulling back the string. This could arm the crossbow in a single motion.
- Cranequin: This device used gears and a crank to wind up the bowstring, requiring multiple turns to pull it all the way back.
- Windlass: Like the Cranequin this was a winding device, but it also came with a bulky pulley system that attached to the back of the crossbow. As a result, this was a tremendously powerful pulling system at the expense of needing a lot of time to arm.
Types of Crossbows
Not Ballistas: Despite visual similarity's ballistas are not scaled up crossbows. Unlike Crossbows which store energy in a set of arms which are bent back, Ballistas instead store energy in twisted rope that has a wooden beam pushed into it which is then twisted back farther before firing to store energy.
Chu-ko-nu: Otherwise known as the Chinese repeating crossbow, this is one of the more common types of cross bow seen in fiction, the term "repeating crossbow" brings to mind some bastard combination of Assault rife and crossbow, which it kinda is and is not. Without of doubt this thing could fire quickly, trained soldiers could loose ten bolts in fifteen seconds however because of how it was fired, the pumping action that pulled the string back also fired it, you could not pull the string back and then aim, you were limited to hip fire giving you almost ork levels of accuracy. Additionally because you were pulling the string back one handed you had a lot less penetration power then a normal cross bow. However the bolts were often poisoned to make up for it (which interesting the Drow in DnD often do), which is not good because no mater how strong the posion on your bolt is, you're not gonna kill a man instantly with a scratch.
Polybolos: also known as a repeating ballista,the Polybolos is more of a big cross bow since the arms are fixed to the "stock" and don't twist around to fire it. The repeating Balista was fired by turning a wheel connected to a chain back to cock it, the turning it the other way to load it again.
Bullet Crossbows: These were essentially the same as regular crossbows, except they fired stone or lead shot instead of bolts.
Crossbow pistol: Small crossbows designed to be fired from one hand, commonly featuring a more modern pistol grip and trigger. While these did exist, they were nowhere near as lethal as battlefield crossbows as the draw weights were far smaller.
Gastraphetes: an early greek crossbow, the Gastraphetes, or "belly bow", was cocked by resting the stomach on the and pushing down so that more energy can be stored then an archer could.
Sauterelle: in world war one soldiers had a problem, all the artillery that existed was big and unable to be drop explosive accurately at close range. The answer was that development of small infantry mortars to allow close fire but as an intern solution the French invented the Sauterelle, french for grasshopper, a Crossbow built to lob grenades.
Arbalast: As European armor improved with the development of steel plate crossbows with wooden bow sections were just not cutting the mustard anymore, as such they began making that part out of steel. This meant that the bow could store more energy and launch a projectile farther and faster, significantly improving armor penetration but it also increased the amount of energy required to draw it, often requiring various leavers and cranking mechanisms to reset it.
Compound Crossbow: the modern version of the cross bow which is basically a modern compound bow on a rifle stock.