"Anti-Town" is a term used to describe play or players that hinder the Town's ability to achieve their Win Condition of hunting down the scum. For example, lurking, self-voting, and claiming without reason are usually considered to be anti-Town behavior. There is often debate in a mafia game about the question of if someone's anti-town behavior is inherently scummy - that is, if it implies that the person acting in an anti-town way is more likely to be scum because of it, or if it's just a sign of poor play dragging the Town down.
In addition to killing all the pro-Town roles, most third-party anti-Town roles also need to kill off any other anti-Town players in the game. The most common third-party anti-Town role is the Serial Killer.
Anti-Town roles or factions are commonly referred to collectively as Scum.