LFG is short term for Looking for Group, which simply implies that one is looking for a group. Groups are usually put together to tackle tasks that would be impossible to solo - many quests recommend more than one player to complete and instances are notorious group activity.
With The Burning Crusade, Blizzard added a LFG tool. This allow the players to put themselves in LFG mode, where you can select what you're looking to do, so that other like minded players can find you. Using the LFG tool also gives you access to the LFG channel, which is a global channel that allow for better communication and coordination.
Close associated is the expression LFM, short for Looking for More - used over the LFG channel mostly. It's a pretty flexible expression that is used to convey that you are looking for more players, i.e 'LF2M' would means you are looking for 2 more players.
|This page should contain additional information on the LFG tool itself and how to use it, as well as arguably the means for putting together groups the old-fashioned way.|