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.
Using the social networking site MySpace to create a clash between cliques/friends using a series of rumors by using the 'bulletin' and 'blog' feature
If John Doe, a friend of Fred Doe, was mad at Fred, he would spread a rumor through a bulletin on MySpace to start a MySpace War, whether or not he had an intention of doing that. A rumor he could spread was that Fred was hitting on Jane Doe, who was the boyfriend of Kevin Doe, who is the captain of the football team. Fred Doe would then spread a rumor back that John Doe actually was hitting on Kevin Doe himself when they were intoxicated at a party that did not exist. The rumors go back and forth until the conflict reaches a climax and deteriorates, if it ever does.