People who try to make your life miserable on a MMO. Good greifers will still make your life miserable but you can still laugh at it. Bad greifers would be people who enjoy making your life suck and only the person doing the griefing finds the act funny.
On City of Heroes, come good griefers are Catharctic, Imeco, Sun Splitter, Forgotten Guardian, Phoenix of Syrinx, Cpt. Mule, and Fo Shizzle; where as the total dickhead griefers are people like Beef Cake, Grandpa Cath, Cyber Wolf Man, Kickblast, and Poser Cath.