Oh whoop, yeah. Let's see...
So you've got a bunch of people (you always do). Of those people, a few of them are mafia players. Their goal is to kill all the town aligned players (all the non mafia players) without getting caught. Everyone's true roles are kept secret, you have to assume that everyone's town aligned. There's a day phase and a night phase. During the day phase, town members (including the mafia) vote to lynch(kill) someone who they suspect may be a mafia player. Some roles may have certain abilities, especially in forum mafia where roles can be hand crafted (like I plan on doing), for example there may be a doctor, who is town aligned and may choose 1 player to save each night.
At night, all the mafia players vote to kill one town player. In forum mafia, a discussion thread is provided (usually quicktopic) for this to be done in secret.
That's the basics, I think?
Usually IRL games last like, 20 minutes more or less depending on the number of players. Forum games can last up to days, again, depending on number of players.