Mafia is a game with two basic teams: the "town" and the "mafia." The town is always the larger of the two groups (typically 2.5-3 times the size of the mafia), but they suffer from a lack of information. In general, no member of the town knows any of his teammates, and is therefore put into a position where s/he cannot trust anyone. Each townie is inherently dangerous, because they have a vote, which they can cast at any other player. If a majority of players vote for a single player at any time, that player is lynched. S/he is then "dead" and can no longer participate in the game.
The mafia members, on the other hand, do know the identities of each other member, which gives them enormous potential power. They will never be put into a position where they must risk losing a team member unknowingly. In addition, they have the ability to Nightkill a player.
The game is divided into Days and Nights. Days are the times when conversations in the thread take place. Any player may generally post any opinions, ideas, or lies they feel like posting, as long as they are within the guidelines set forth by the mod. This may include accusations, arguments, defenses, speculation, and most importantly, votes. Once a player is lynched by a majority, the Day - which may take weeks to conclude – ends. The game enters a period of Night, during which the mafia may converse via PM, and the mod will manage any abilities, such as Nightkills. The mafia’s conversations will eventually end with a decision as to who to Nightkill; the final choice is sent to the mod. That player is then "dead" and may no longer participate.
Any number of other abilities may also be used, though these are specific to each game. One of the more common roles with abilities include Cops, which can investigate a player each night by sending that name to the mod. The mod will then respond to that player with a result of Innocent or Guilty – an indicator of whether or not a player is a member of the mafia. Another common role is the Doctor, who can send in the name of a player at night with the hope of defending them. If that player would have been killed that night, the Doctor's ability prevents the kill.
Many Mafia games have themes that make them very interesting to play in. Game mods often go to great length to make games saturated with flavor, which often bleeds into game mechanics. Thus, every game is different, and some of them are very complex. Strategies and counter-strategies have been developed for hundreds of situations, and yet, the thrill of uncertainty that made the original game so exciting to play is still present. Go sign up for a game, and see what the craze is all about!