Well, they are mostly two players with different affiliation (mafia and a townie for instance), and their goal is to live through the whole game until they become the sole majority and this is how they win too. So they need the players pool to be decimated until a total of four and being among those four to win. The success rate that this happens is pretty low though.
... I know the roles (even if I can't remember some of them), but I want to know how some of the came about. They just look weird, like the parnaoid gun owner or serial killer.