Ork Boy
"Da first an' best part of a WAAAGH is a good ol' mob of Boyz!"
The Ork Boy is the most ubiquitious type of Ork in the entire galaxy, and properbly the most common warrior in Warhammer 40000. A Boy is basically your average Ork, a brutish war-addict, who has no concept of "enough" - Every Ork will always strive to be better at their trade (Read: Killing) than the rest of the Boyz in the Mob by getting bigger weapons, harder armour and by generally being more boisterous than the rest. The vast majority of the Ork race are Boyz in some capacity and any proper Warboss has thousands of these under him, only waiting to get into a scrap - To most of the Boyz, it doesn't matter where or with who, just as long as they get to fight something.
Most Boyz are just happy fighting, shooting and bludgening, but a some stick out from the norm and feel attracted to a certain type of fighting. Examples are Burnas, Stormboys and Tankbustas, who are called something else by other Boys because of this tendency. Some even get urges to do stuff that isn't strictly fighting, which understandably seems fishy to most Orks, and are therefor called Oddboys.
The mobs of Boys are lead by Nobs, who keep the Orks in line and krump them if they get uppity. He also serves as a means of bringing orders to the mob from the Warboss.
Armaments
Most Boys like smashing as well as shooting stuff, so it's up to the individual Ork to get what equipment he wants. A handgun of sorts called a Slugga are usually a good pick for the Ork dispositioned to melee, as he can get a few noisy shots of before going into krumping range, while the more shooty Orks pick up Shootas, which spew out a lot more shots than a Slugga and is noisier to boot. Most Orks also wear a simple leather overall with some metal plates for protection (which can't protect them at all no matter how much that Ork believes it can). A few brings heavier armour though, which can stop most normal bullets. Most also bring a few Stikkboms for the occasional bang.
Tabletop
The standard Ork Boy is the workhorse of almost all Ork forces and are some of the best horde infantry in the entire game. Bought in units of 30 Orks at most at 6 pt per Boy, you can quickly and very inexpensivly get huge numbers of models on the table, able to swarm your opponent and fuck over any plans he had of Deep Striking something behind your lines, simply because there won't be any space. They look cool too, having numerous options and being a well of inspiration for kitbashing, and a tide of green muscle just feels so, so... Warhammerish.
Bear in mind that in 7th Edition, Boys don't work as they have always done, simply because so much can kill your green hooligans before they get into charge range - To get that far, your Boys have either used valuable time digging through the dirt to get some Cover saves or been running through the field getting shot at, while probably suffering numerous Leadership tests, who can be circumvented by the Mob Rule, but as that will make the unit hit of lumps of itself in the process, it isn't always favourable to do so. When (or if) you reach the enemy line and get to bashing, your mob might be severely depleted, and then you have to get through Overwatch and pretty much anyone hit before you in melee, so you might be skewered by those lasgun-bayonets before even getting to krump things.
This is where the new (or old) WAAAGH-rules come into play. All Orks now have 'Ere We Go, which let them reroll one dice when charging, which is pretty good in itself, but it harnesses something far better - If you have a Warboss in your force (And why wouldn't you), you can call a WAAAGH, which lets all units run AND charge in the same turn. This lets Boys able to reach combat in turn 2-3, which is frankly invaluable for a force based so much around getting stuck in with a wicked amount of models as the Orks. On top of that, all Orks have 2 Attacks, and get one if they have Sluggas and Choppas, alongside that crucial extra Attack from charging. And they all have Furious Charge as well, which, while rather meager in this version, help them krump gitz even moar proppa - Just remember that they have to get there somehow, and that isn't something you can do lightly - Orks are random and fun, yeah, but you can really get somewhere by thinking a bit, as with Warhammer 40,000 in general.
Your average Boy has Stikkboms and a Slugga and Choppa, and from there you can upgrade to your hearts content. For a measly 1 pt you can get a Shoota instead of the Slugga, losing one Attack but getting an Assault 2 S4 AP 6 gun in exchange. This helps soften enemies up before mopping them up (Shoota Boys can still kick all kinds of ass, you just need to make sure the unit they charge is softened up enough before charging), but can also put enough shots downrange to be effective as a ranged squad. You can also give any Mob 'Eavy Armour, a cheap 4+ Armor Save, but think about this before upgrading the whole bloody Mob - 4 pts per Boy is a lot when you will likely have around 30 of them running around with it, and there are a good amount of weapons who can ignore 4+ saves in the game right now.
You can also give up to 1 boy per 10 a Special Weapon, which is either a Big Shoota (A nifty weapon for putting more shots downrange, but not strictly needed) or a Rokkit Launcha (A cheap S 8 AP 3 a turn), which can boost the units shooting. Most important is now the Nob, who can help the squad out in combat as well as with Mob Rule checks. He has all the standard options for a Nob - Kit him out to help the unit.