Exor will try to explain a "mafia game" as best he can.
So let's say there's around 25 players (well, it's an example, a mafia here would have less. Depends who's interested). About 5 of them would be mafia, and the rest are town (or a few independent, but let's focus on town and mafia for now). A mafia member knows the identities of the other mafia members, and may talk to them in a Quick Topic, but the town doesn't know the alliance of anyone.
There are different phases, each lasting about 24 hours usually. During the day phase, people vote for who to lynch that day. During the night phase, people can use unique night actions and the mafia may kill someone.
Town goal: To eliminate all mafia
Mafia goal: To outnumber (or sometimes eliminate) the town
There are usually a few independents as well, each with their own win condition.