One of the graddadies of the grid based first person dungeon crawler, and one of the best.
There is no campaign. Hell there's not really any game here to speak of at all. There is no story beyond the fist cutscene. In other words, there is nothing here but a pack of tedious map missions with no indication of which a beginner should try first. If you are fine playing just map missions and multiplayer, then I'm sure you might possibly like this game.
Postal 2 has some 0k humour in it, there are bits that are quite funny, but the thing that really defines Postal is the endless boredom of it. You go somewhere, you cause havoc. Big deal. Mowing down unarmed pedestrians just feels like a waste of time. There is nothing whatsoever to get you invested in this game in any way. The game tries its best to offend you, but ultimately it's just offence for the sake of it. It can't offend you because the sheer amount of offence makes the offence nothing but routine. You can't even vent your anger in this game because the game world itself is utterly apathetic! Go ahead, the game says, cause a massacre in the street, set people on fire. The whole thing will be repopulated with clones two minutes later. And that is the game's biggest flaw. It just doesn't inspire any emotion. Nothing you do in this game matters, and so there is no reason to do it.