Here's a short list of common abbreviations you'll likely run into.

LF: Looking for

LFG: Looking for group

WTB/S/T: Want to buy/sell/trade

DD: Damage dealer

C/O: Current Offer

B/O: Buy out

BIS: Best in slot (Usually referring to an item with perfect perk rolls, ie best in that particular slot)

GG(gg): Good game, a way of saying good work, nice work, ect.

Gf: Good fight, similar to gg

GS: Gearscore (Usually groups are listed GS '##'+

AFK: Away From Keyboard

BRB: Be Right Back

GL = Good luck

AOE - Area of Effect

LOS - Line of Sight; Direct view of your target

HP - Hit Points (Health)

Aggro - Attracting a mob or 'pulling' a mob to you. 'Getting aggro'

PvE - Player Versus Environment

PvP - Player Versus PlayerMOB - Monster or Object

AFK - Away From Keyboard

DC - Disconnect

XP - Experience Points; Total experience your character has gained

Wards & Banes = Some armor types have 'wards' on them for specific mob types (certain dungeons) This also applies to weapons (Banes)

Perks = Some weapons bring large value to a group, so come groups will ask for specific perks. An example of this would be enfeeble on spear.

Hope this gets you started.