A term originally used to describe someone who constantly derides others success, can be on an individual basis or directed towards an organization or sports team.
Most common though, the term is used by fans of organizations whose actions are unethical or illegal enough to warrant such "hating", and accuse people of being haters instead of actually defending their team.
Jets Fan: The New England Patriots are cheaters, they should have their titles taken away!
Patriots Fan: OMG U A HATER!
Bears Fan: The Indianapolis Colts are a bunch of softies with bandwagon fans. They're a stolen team whose president changed the rules just so passing is easier.
Colts: NOT LISTENING CUZ U A HATER
Bears Fan: Brett Favre is overrated. All he does is throw interceptions in playoff games. He needs to retire, because he's just playing for personal glory now.
Packers/Vikings: U JUST A JEALOUS HATER!
Ravens Fan: The Cleveland Browns suck and haven't won anything in forever.
Browns Fan: Yeah, they suck, what's your point?
Ravens Fan: You're not going to call me a hater?
Browns Fan: Why would I?