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.