Wartrakks are (or were, as of 8th edition) one of the most commonly used Ork vehicles. Cheap and relatively effective, they are essentially a Warbuggy mounted upon a sturdy track unit; a Wartrakk sacrifices some speed for the ability to traverse rough terrain such as a ruined city and deliver heavier firepower. It should be noted that the model is relatively outdated compared to... the entire rest of the army list, excluding the Warbuggy. This shows through with the wheels and the orks themselves, both of which being a bit more bland than other ork vehicles. This sort of thing is present in other armies too, but considering the general quality of the rest of the Ork List it is especially noticeable here, with the old models being extremely fucking boring. Kitbashing is STRONGLY recommended.
Usually most Ork warbands are accompanied by a ramshackle band of Wartrakks that hurtle around the battlefield shooting at anything they can. Most Wartrakks come with a twin-linked Big Shoota which makes it great for both fast assaults and clearing light infantry. As of 7th edition Wartrakks and Warbuggies can get a twin-linked Rokkit Launcha for free. Cue endless Lulz as you have a relatively fast anti-tank platform. They are also relatively cheap so you can try and spam those if you can. Of course they are still as lightly armored as the Warbuggies so a simple swift breeze can dismantle the entire thing.
As of 8th edition, the old-as-balls Wartrakk has been removed from the Ork army list.
Essentially the Ork's take on the Imperial Hellhound, the Skorcha version of the Wartrakk utilizes twin "Skorcha" heavy flamethrowers. Fueled by a large tank of promethium, Wartrakk Skorchas specialize in incinerating the enemy with great sheets of flame. The crews of these vehicles are usually Burna Boyz for whom man-portable burnas just aren't enough. Usually they have a Gretchin to mend the promethium tank so that it doesn't overheat and blow up in their face.