Oi, ya git! Get in, wese gonna have some fun!
Trukks are the very basic vehicles used by da Orks in Warhammer 40,000. While GW emphasizes that it's a specific vehicle, in reality it can be any Ork vehicle smaller than a battlewagon. It is by far the most common Ork vehicle, used by every force from the most basic Boy to Warbosses and Goffz to Evil Sunz. Most commonly they're the barebones chassis of some looted vehicle, an engine, and whatever scraps of metal or weapons are bolted onto it. Usually they're owned by a Nob and used to carry a mob of Boyz to the front.
If a Mek (or any Ork or neckbeard in general) gets their grubby mits on the burned-out shell of a Rhino, Ghost Ark, or any kind of small transport, it will most likely be kitbashed into a trukk of some variety. Given the Ork fluff, simple rules in terms of crunch, and the lenient "counts as" rules at most tournaments when it comes to Ork vehicles, the trukk has the most variety and customization potential out of any vehicle (and maybe any model) in the entire game.
The trukk can be taken as a Dedicated Transport for nearly any Ork unit outside of Kommandos and Grots. For a 12 man open-topped transport with AV 10 all around and 3 HP, it is a steal at 30 points. Seriously. Grots are more expensive than one of these things.You have the choice between a Rokkit or a Big Shoota as standard, or slap a Wreckin' Ball on there for shits and grins. They come with the hilarious rule Ramshackle; making Penetrating Hits simply Glancing on a roll of a six. Laugh your ass off as your opponents Lascannon takes off nothing more than a piece of sheet metal.
Besides the Reinforced Ram and maybe the Boarding Plank, there should be no reason for you to get anything more than the bare chassis of the trukk. They're meant to be cheap dispensable transports for your Boyz and nothing more. If you choose to bring a trukk as a Fast Attack vehicle instead of a transport for your guys, consider yourself retarded to the highest degree. (Unless its a 750- point game where you wanted to fuck around and give a trukk to some Gretchins or some shit.)