A player in an online multiplayer game whose goal is to be as absolutely obnoxious to other players, typically for teh lulz. This is traditionally actions such as team killing, team wounding, blocking, and spawn camping. Note that it is a misconception among a large part of the internet (particularly care bears, noobs, and nublets) that complaining to or about a griefer will deter them. In fact it is the opposite; this is the ultimate confirmation that they are annoying other players.
"LEAVE! LEAVE BEFORE THE ADMIN BANS YOU!" - Konane
"OMGGGG ADMIN BAN THE TKER!!!1!!111!!! HES A GRIEFER!1!!" - n00b who thinks there is an admin online