was a Death Eater, but he turned on Voldemort and began to spy for Dumbledore. He is an Occulumus and it enables him to close off emotions in his mind that allow others to know if he is lying or not. They don't explain why or how Snape turned but I think it will be dealt with next book. Know what I don't know is how Malfoy and the other Death Eaters didn't find out that Snape was a spy back in the hayday of Voldemort. In Goblet of Fire Harry sees Dumbledores thoughts in the Penseive that are about the trial of Karkaroff where it is said that Snape has turned on Vorldemort and is working for the Ministry. Malfoy has all kinds of pull at the Ministry, at least before the end of Book 5 he did. How did he not know that Snape was a traitor? The problem is that it's alluded to that Snape is in contact with the Death Eaters finding out what Voldemort is telling them. Why wouldn't Malfoy and the rest have already killed Snape for his betrayal a long time ago?
www.hp-lexicon.org has some timelines and they put Tom Riddles hogwarts days being in the 40's while Snape & James Potter and crew were there in the seventies. It's a pretty cool site, but it's not really official I suppose.