-RP - RolePlay, Role-Play, Role play:
That is what it's called when you act out a character that has a different reality than your own.
-RL - Real Life:
It's just a myth, but some say there is this world outside the computer where a giant ball of fire called Sun burns in the sky and beings of a fleshy nature practice something they call 'physical interaction'. Scary!
-OOC - Out-Of-Character:
Out-of-character is the term used for things that are said or done not in the persona of your character, but your own persona. It is when you, the player, say or do something, not the character.
-IC - In-Character:
In-Character is the term for things that are said or done by your character, usually with no relation to your Real Life persona.
-Resident:
Resident is what every person in SecondLife is referred to as. We are all Residents.
-AV - Avatar:
A virtual representation of a person or a person's interactions with others in a virtual environment such as SecondLife, conveying a sense of someone's presence (known as telepresence) by providing the location (position and orientation) and identity; examples include the graphical human figure model, the talking head, and the real-time reproduction of a three-dimesional human image.
-Alt or Alts:
An Alt (from alternative) is a character/AV that is not your main character/AV.
-GM - Game Master:
These are the guys with god-complex, who rule over the lives of others in the virtual world.
-Admin - Administrator:
Administrators have one of the worse jobs in RP sims: They enforce the rules, resolve disputes, help coordinate events... and on top of that nobody tends to like them.
-PC - Player Character:
Your character is a PC (even if you are using a Mac).
-NPC - Non-Player Character:
A non-player character is a character that is controlled by the GMs. NPCs are used as tools to interact with players usually at crucial points of the storyline, or simply to fill in a token role.
-Godmoding:
Godmoding happens when a player forces an action (and sometimes a reaction or outcome) upon other player(s). For example, Bob and John are in a bar fight, and Bob decides he will hit John with a chair:
"Bob grabs the nearest chair and swings it at John, hitting him in the back and breaking his spine."
In that pose, Bob not only didn't give John a chance to evade the attack, but also determined the outcome, which in this case is pretty severe. Only GMs can take this kind of action, rarely on players but usually on NPCs, and still it pisses people off. So don't do it.
-Metagaming:
Metagaming is a broad term usually used to define any strategy, action or method used in a game which transcends a prescribed ruleset, uses external factors to affect the game, or goes beyond the supposed limits or environment set by the game.
In simple terms, using out-of-game or OOC information, or resources, to affect one's in-game situation. It pisses people off, so don't do it.