You can read the rules here. It is a bit lengthy so new players aren't expected to memorize it by heart, but this page will capture the gist of it.

Game Violations

  • Cheating. Don't cheat with your friends or with multiple accounts.
  • Copied Mechanics. You aren't allowed to copy-paste in-game system messages or meetings, because then it'd be too easy to prove your role.
  • Game throwing. This refers to intentionally not playing to win, often done to antagonize another player by preventing them from winning. Never give up. Keep playing to win.
    • Rare case: continually no-lynching or no-killing when you have a chance to win. See: Meteor.
  • Insufficient Participation: This is for if someone doesn't really talk. Mafia is a text based game, and as such you have to contribute to the discussion.
  • Outside Game Influence: Things like posting information from a game in progress on profiles, lobbies, or the forums, clearly stated meta posted on your profile, reporting a player for a game in progress, using third party or sites to make in-game decisions, bribes or threats (such as karma, kudos, and reporting), and pregame pacts.
  • Game Related Suicide: Leaving a game for game related reasons, like trying to avoid a loss.
  • Trolling: Disrupting the game. This includes antagonizing other people, and actions that purposefully disrupt the game.

Those are the big ones you should know before playing a game. Here are the rarer violations:

  • Encouraging Rule Breakage
  • Repeated Suicides
  • Spamming
  • Lobby Camping
  • Lobby Trolling

Community and Site Violations

  • Harassment
  • Hateful Comments
  • Outing of Personal Information

Those three are the main ones you should worry about. The others can be summarized:

  • No X-rated or inappropriate content
  • Don't spam the site
  • Don't try to exploit the code
  • Don't try to bypass suspensions

When do the rules apply?

Game violations only apply to ranked and competitive games in the Main lobby. It is still common courtesy to not break the rules in unranked games, but they can't be punished

Community and Site violations apply all across the site - on the forums, on user profiles, reports, etc.

Okay, so how are these rules enforced?

See: Moderation