Thanks to Blizzard Support for this info!
Add: Will usually refer to any additional monsters that have started fighting your group after you are already engaged in combat. For example, if you are fighting a bear and half way through the battle a wolf comes along and begins attacking you, this would be considered an "add".
AFK: Is short for "Away From Keyboard". You can active the AFK flag on your character by typing /afk in the chat line. It also automatically activates if your character is idle for an extended period of time
Aggro: This refers to aggressive actions against a player. If a monster turns and attacks you, you have gained aggro.
AH: Is short for "Auction House". This is where you can sell and buy items from other players of your same faction (and opposite factions at Gadgetzan).
AOE: Is short for "Area of Effect". This is when a spell can affect all targets in an area instead of being directed at a single target. Blizzard, Frost Nova, and Hellfire are all examples of AOE spells.
BB: Is short for "Booty Bay".
BFD: Is short for "Blackfathom Deeps".
BOE: Is short for "Bind On Equip". This means that when you equip the item, it will no longer be tradable to another player and can only be sold to a vedor.
BOP: Is short for "Bind On Pickup". This means that when you loot the item from the monster, it will no longer be tradable to another player and can only be sold to a vedor.
BRD: Is short for "Blackrock Depths".
BRS: Is short for "Blackrock Spire".
Buff: A spell cast on a player, pet, or mob which enhances a specific statistic for a set time period. Spells such as "Mark of the Wild" and "Power Word: Fortitude" are examples of buffs.
De-Buff: A spell cast on a player, pet, or mob which places a negative effect on a specific statistic for a set time period. Spells such as "Demoralizing Shout" and "Curse of Weakness" are examples of de-buffs.
DOT: Is short for "Damage Over Time". This refers to a spell or effect that slowly deals damage over an extended period of time, rather than all at once.
DM: Is short for "Dire Maul".
DS: Is short for "Darkshore" and or "Darkshire".
GG: Is short for both "Good Game" and "Good Going" depending on the context in which it is used. If you happen to save a party member, someone might say "GG" meaning good going. If you beat some one in a duel, the other person might say "GG" meaning good game.
GZ: Is short for "Gadgetzan".
HOT: Is short for "Heal Over Time". This refers to a spell or effect that slowly heals damage over an extended period of time, rather than all at once.
IF: Is short for "Ironforge".
KS: Is short for "Kill Stealing". This occurs in games when another player or group attacks a mob that you are already fighting in order to get the experience or items from it.
LBRS: Is short for "Lower Blackrock Spire".
LFG: Is short for "Looking For Group".
LFM: Is short for "Looking For More".
LS: Is short for "Lakeshire".
Mara: Is short for "Maraudon".
MC: Is short for "Molten Core".
MH: Is short for "Menethil Harbor".
MMORPG: Stands for "Massively Multiplayer Online Role-Playing Game". In an MMORPG, thousands of players exist in the same game world at the same time. This creates an incredibly rich and active environment in which interesting things are constantly taking place. MMORPGs provide thousands of hours of game play, with a nearly infinite variety of goals to achieve across a vast world covering miles of land and sea. For the World of Warcraft, we will also be adding new content on a regular basis, thus ensuring that there will always be new adventures, new locations, new creatures, and new items to discover.
Mob: Is short for "Monster" and is a word used to describe creatures that you kill in the game aside from other players.
NPC: Is short for "Non-Player Character". These are computer-controlled characters. Quest givers, merchants, inn keepers, etc.
OG: Is short for "Orgrimmar".
Proc: A term used to denote when a special effect of an item randomly triggers or what the special effect of said item does. Proc only refers to a randomly triggered effect that arises from attacking or being hit. If your item has an ability to click it to make it cast a spell 100% on demand, that is not considered a Proc.
RFC: Is short for "Ragefire Chasm".
RFD: Is short for "Razorfen Downs".
RFK: Is short for "Razorfen Kraul".
RR: Is short for "Redridge".
Scholo: Is short for "Scholomance".
SFK: Is short for "Shadowfang Keep".
SM: Is short for "Scarlet Monastery".
Strat: Is short for "Stratholme".
TB: Is short for "Thunderbluff".
Temple: Is short for "The Temple of Atal'Hakkar."
Train: Train or training is when someone pulls a group of monsters past another player, causing them to get attacked. Please do not confuse this acronym with the training you do at your own class trainer.
UBRS: Is short for "Upper Blackrock Spire".
UC: Is short for "Undercity".
Uld: Is short for "Uldaman".
VC: Stands for Van Cleff and is used to refer to "The Deadmines".
WC: Is short for "Wailing Caverns".
WTB: Is short for "Want To Buy". Usually used on the Auction channel. Example: "WTB stack of 10 light leather"
WTS: Is short for "Want To Sell".
WTT: Is short for "Want To Trade".
XP: Is short for "Experience Points".
ZF: Is short for "Zul'Farrak".