AC - Armor Class or Defense.
Add - An extra monster has joined an existing battle.
Alt - A character on your account other than your main character.
AoE - Area of Effect Damage. This is a spell that hurts a group of monsters in an area.
AE - Area Effect
AFK - Away from Keyboard. This means the player is away from his computer.
Aggro - This means the monsters are mad at you and you've "activated" them to attack you. They are now in the motion of trying to reach and attack you.
Aggro Radius - The radius around the monsters where they will "wake up" and attack you.
AGI - Agility
AH - Auction House
Avatar - Your character
BG - Battleground
Buff - A beneficial spell cast on a monster or player. An example of a "buff" is the Priest's Physical PDef buff.
Carebear - Player that prefers to help other players attack monsters rather than attack other players in player vs. player combat.
Caster - A character that stays on the back row to heal or cast spells on the enemy, such as a Mage.
Char - Your character.
Cheese - To exploit an imbalance in the game.
Creep - Monster
Creep Jacking - A term from Warcraft 3 where players attack other players while they are already engaged in combat with neutral monsters.
Critters - Monsters that don't attack back.
DD - Direct Damage. This is a spell that does all of its damage in one hit rather than spreading its damage over time.
DMG - Damage
DOT - Damage over time
DPS - Damage per second
De-Buff - A negative spell cast on a unit that makes it less powerful. An example of a "de-buff" is the Werefox skill "Soul Degeneration".
FB - Fuben (= Chinese for dungeon), the dungeon quests you have to do at higher level but which you receive at level 19, 29, ...
FH - Full Health
FM - Full Mana
FTL - For The Loss, a sports or game term.
FTW - For The Win, a sports or game term.
Gank - Player vs. Player: To attack another player while they're trying to fight a monster. Or...when a party attacks a lonely player, usually to PK (Player Kill) him.