For those who aren't yet very familiar with roleplaying and the jargong that sometimes pop up, here's a list of things that you might run into:
AC - Armor Class
AFK - Away From Keyboard
AU - Alternate Universe
AV - Avatar
BAB - Base Attack Bonus
BRT - Be Right There
CR - Challenge Rating
d# - # Sided Die
D&D - Dungeons & Dragons Roleplaying Game
DFTBA - Don't Forget to Be Awesome
DM - Dungeon Master
GM - Game Master
GURPS - Generic Universal Roleplaying System
HP - Harry Potter or Hit Points
IC - In Character
IRL - In Real Life
MMORPG - Massively Multiplayer Online Roleplaying Game, usually a video game.
Mini - Miniature
NPC - Non-Player Character
NWoD - New World of Darkness Roleplaying Game
OC - Original Character
OOC - Out of Character
OWoD - Old World of Darkness Roleplaying Game
PBP - Play By Post
PB - PlayBy or ProBoards
PC - Player Character
PNP - Pen and Paper
RL - Real Life
RPG - Role Playing Game
ST - Story Teller
WC - Word Count
WoD - World of Darkness Roleplaying Game
WotC - Wizards of the Coast
Alternate - A character that you play less often than your main/primary character.
Alignment - A way of describing your character's intent, such as "Lawful Good."
Bio - Another word for a character profile.
Canon Character - A pre-existing character from a specific world, such as "Ron Weasley" in Harry Potter.
Character Sheet - A profile or list of important information and stats for a character.
Dungeon - Part of a traditional or video game where characters go to slay monsters and complete quests.
Dungeon/Game Master - The person who leads or runs a game, especially used in tabletop games.
Fade to Black - A posting technique in which action fades, leaving what happens next up to the imagination.
Familiar - An animal that is an intelligent close companion to a character.
Fandom - The group of followers or the genre based on a specific pop culture item or icon, such as Doctor Who.
High Fantasy - A genre involving lots of invented elements, like in Lord of the Rings.
Initiative - Usually used in traditional games, a way of deciding which character goes first.
Journal-Based - Often used on LiveJournal, a game where threads take place in the journal of each character.
Low Fantasy - A genre involving few invented elements, such as elves in a modern day world.
Lore - The background story or information about a particular universe.
Literate - A word used to describe someone who can read and write coherently.
Main - The primary character that you play most often, more than any other.
Mary-Sue or Gary-Stu - A "stereotypical" character, often over the top and perfect with very few flaws.
Miniature - A small figurine used in tabletop gaming to represent your character.
Non-Player Character - A character that is only mentioned in passing, such as a barmaid.
Off-Topic - A post in a thread or forum where it doesn't belong, such as posts about Batman in a Glee thread.
One-Liner - A role playing post consisting of little more than one sentence.
Original Character - A player-created character that does not already exist or is not based on another character.
Pen and Paper Game/RPG - A traditional game played offline, usually with dice, such as Dungeons & Dragons.
Period Game/RPG - A role playing game set in a specific time in the past, such as the Tudor Era or Medieval Era.
Play-By-Post - A type of role playing done by posting in a forum.
Play-By - A celebrity or model chosen to represent your character.
Player Character - A fully fleshed-out character that is actively in play by a member of the game.
Real Life - A type of game with little to no fantastical elements, based on a normal world.
Tracker - A list of things for easy reference, such as threads someone is participating in.
Traditional/Tabletop Game - Another word for an offline, pen-and-paper roleplaying game, or wargame.
Turn-Based - A style of play where each player posts in a specific, established order.
Verse - Usually follows a word or descriptor like Buffy or Potter to refer to that genre's world.
Wargame - A strategy game played with miniatures where players act out battle.