Severus Snape is the son of Eileen Prince, a Slytherin, and Tobias Snape, a Muggle. While Severus was at school he named himself The Half-Blood Prince because he was ashamed of bearing his father's name. He was a Potions teacher at Hogwarts then he became a DADA teacher. Apparently he killed Dumbledore but no one knows why and no one knows who's side he's on.
I'll see if i can find out anymore later. Afraid my brain has gone dead now. It's been a long day...