There are many definitions...one being when two friends exchange login information in the past, get into a fight (minor or major or playful) and go and change everything on each others myspace.
a girl, "Sarah," and a guy, "Peter," get into a MYSPACE WAR. They already have each other's login information. "Sarah" puts a Barbie layout and girly pictures of herself on "Peter's" profile. Meanwhile, "Peter" has put animal pictures and pictures of HIMSELF on "Sarah's" profile. They also are posting humiliating bulletins about themselves.
--it's quite funny, actually.