Here is a detailed explanation of the forums. Please reference this guide if you are unsure of where your post should go. First and foremost, some definitions:
In Character (IC)
In Character speech is dialogue written from your player account's point of view. Remember that your player is living in the golden age of the mafia, from the years of the late 1920's to the early 1940's, and this game is primarily based on the mid 1930's. Stay in the general timeframe, however and you'll be alright.
Out of Character (OOC)
Out Of Character speech is dialogue written from your point of view, and not your character's. This should be simple enough to follow.
In Real Life and In Game (IRL and IG)
This should be an easy concept to comprehend, but often times it isn't. IRL has nothing to do with game. IG has nothing to do with IRL. IRC is not IG. If you choose to carry out IG meetings on IRC, that is your choice, but be very aware not to blur the lines between things that happen in an IG context to that which affects you IRL. If you character is killed in game for actions that happened in game, do not attack the PLAYER who killed your CHARACTER with personal attacks IN REAL LIFE.
Role Play (RP)
Role Play is the act of playing a part, and in this game refers to your character assuming the role of an early 20th century citizen of the six cities in the time of the mob culture. It is akin to acting, only in the textual sense.