For the details of the game itself, see Mafia (game).
The Mafia is a group of players that seek to gain majority within the town through killing members of the Town during the night, and avoiding the Execution during the day. They know who their comrades are, and typically have a leader, usually a Mafia Godfather, to lead them. They are free to speak privately outside of the game they are playing in. Mafia members are vulnerable to Cop investigations as they turn up guilty. They are also vulnerable to Serial Killers and vigilantes who target their members for kills.
Players often refer to members of the Mafia as "Scum," although the term can be applied to any faction not aligned with the town.
Common Mafia Roles
Mafia Godfather: Usually the leader of the Mafia group, the Godfather also is immune to cop investigations and will investigate as innocent. The Godfather is usually in charge of assigning night actions to his members.
Mafia Roleblocker: A Mafia-aligned Roleblocker. This role is important for the Mafia group because it is the most effective way of blocking Cop and Doctor night actions, and is frequently coordinated with the nightkill to effectively silence town power roles.
Rolecop: A member of the Mafia that can investigate other players and discover their roles. The exact working of the Rolecop varies from game to game; some are allowed to see their target's powers, while others only get a flavor name.
Bus Driver: A role that switches two players at night, so that actions targeted at one of the switched players will affect the other instead. This role is designed to cause confusion among the town and also to throw off town power roles.
Framer: A role that frames one person at night, causing that player to show up as Guilty on a cop investigation.