*CHOO CHOO* OI! DA RAPE-TRAIN IS A'COMIN'!
Now that we've gotten that out of the way...
The Battlewagon is the Ork equivalent to a Space Marine Land Raider in terms of both size and firepower, although Orks being Orks, it is not quite as durable. Basically the most badass and ramshackle pimpmobile ever. Seriously, you'll never see another vehicle come in more lovely shapes and sizes than a Battlewagon. Its typical role is to ship mobs of Boys to the front while also carrying ridiculous Orky gunz. The most common main gunz are the
Lifta-Dropa (now OOP) Zzap Gun, the product of a one night stand between a lascannon and a plasma cannon; the Kannon, your standard boom-gun that can fire either piercing shells or frag rounds; the Killkannon, which has shorter range than the Kannon but is potentially more destructive; and the Supa-Kannon, which you have to go through Forgeworld to get the rules and kit for, so fuck you. In Ork kulture Battlewagons are status symbols most commonly owned by a Mekboy wanting to have something to carry his loot, or a Warboss and his retinue of Nobs. While each Battlewagon is unique, they most commonly have a giant bulldozer blade and big gun turrets on top. They also commonly feature extra bits such as up to four big shootas, a grabbin' klaw, Stikkbomb Chukkas, wreckin' balls, a Deff Rolla, or even a Lobba (dat's a mortar for youse 'umies). It can also include a Grot Rigger who ensures that the vehicle keeps running through the simple expedient of whacking its internal components with a spanner as hard as he can.
It represents what all Orks are supposed to be: loud, bombastic, and ded killy.
On the Tabletop
sadly meh on the tabletop awesome, but it is very easy to rack up points when outfitting one.
Its main issue is the starting price tag of 120 points. Remember, that is just for a bare bones "stripped" Battlewagon, no weapons or upgrades to it. Also, it can't be a dedicated transport for your Boyz, so you'll have to eat up a Heavy Support slot in order to drive one around. And sure, it is T7/T8 with 16 wounds and a 4+ save, but if your opponent has the right anti-tank tools bad things can still happen. On the bright side, you can stick ridiculous amounts of firepower on this thing.
As of 8th Edition, the Battlewagon comes in 5 distinct flavors. Each variant emphasizes a different role and carries a different starting point cost.
The first variant is the aforementioned "basic" Battlewagon at 120 points, which is simply called--you guessed it--a Battlewagon. And oh yeah, its transport capacity is twenty fucking Boyz. Shit, your opponent will do everything in his power to shoot it off the table before it gets to his gunline, especially if it's full of MANz and a Warboss. This is why a Battlewagon can be a good DISTRACTION CARNIFEX. Though not as viable as it used to be with that price tag, it can be a pretty good suicide missile when equipped modestly. This version also features an Open-Topped troop bay, which means that all Boys riding in the vehicle can shoot outside. However, being Open-Topped makes the vehicle T7 instead of T8. Putting the 'ard top on it is 0 points, but you lose Open-Topped.
The second variant is the Gunwagon at 140 points. This version is 'ard-topped (thus being T8), and it can only carry 12 models. Since this is the "dakka" variant it includes a Kannon and a periscope as standard; the periscope allows it to fire its main gun twice each turn if the vehicle moves less than half speed in the movement phase. The Kannon can be replaced with either a Zzap gun or a Killkannon for no additional points cost. Although it is probably the least used version it can still be a major ranged threat when used correctly. Note that it cannot take a Deff Rolla.
The third variant is the Bonebreaka at 140 points. Like the Gunwagon it is 'ard-topped with T8 and can carry 12 models. However, since it is the melee variant it carries a Deff Rolla as standard equipment, and also gains an additional D6 attacks when it charges. That's potentially 12 S9 AP-2 melee attacks doing 2 wounds each, and oh yeah, the Deff Rolla means it's hitting on 2's.
The last two variants are the Forge World exclusives. The first of these is the Supa-kannon Battlewagon. It's 161 points before adding the cost of the Supa-kannon (+30 points); it's Open-Topped and T7, and transport capacity of only 6 Boyz. Like the default Battlewagon you can put an 'ard case on it, and you should; it'll cost you 3 points but 6 Boyz aren't doing much shooting. Finally, the similar but distinct Lifta Wagon comes standard with a Lifta-Droppa; 175 points and the Lifta-Droppa is free. This version has no transport capacity but seems to be open-topped anyway because it's T7. Weirdly, unlike the Bonebreaka and Gunwagon, these two variants count as BATTLEWAGONs, but unlike those two, they can't take most of the Battlewagon Equipment upgrades.
Aside from each variant's unique ability, the three core Codex Battlewagon profiles can still take nearly all of the equipment that the others can. So if you really want to put a Lobba on your Bonebreaka or a Zzap Gun on your 20-Ork Battlewagon, go for it. This is one of the coolest vehicle models in the fucking game, and still has the best scratch build potential out there. Go out there, and make your own little scrappy fortress on wheels!