An
individual who uses online games, instant messaging, e-mail, or any other communication method to harass, obstruct, or in some
way make an experience
sour for another person.
In IM (instant messaging) a griefer might keep sending spam messages to someone, knowing the messages will overload the other person's
computer.
In an
MMORPG, a griefer might run around a spawn location shooting newbies.
Someone who causes
grief or a bad experience.