In the sixth book, Snape murders Dumbledore.
In the seventh book, Snape shows Harry why he did it. Turns out he loved Harry's mother, Lily, and turned to Dumbledore for help when he found out Voldemort was going to kill her (and her son and husband). When Dumbledore has one year left to live he asks Snape to kill him in order to help others.
Severus Snape: For him? Expecto Patronum!
(A silver doe patronus like Lily Potter's emits from Snape's wand.)
Dumbledore: After all this time?
Lily Potter: Does it make a difference, being Muggleborn?
Snape hesistated. His black eyes, eager in the greenish gloom moved over the pale face, the dark red hair.
Snape: No, It doesn't make any difference.
In the books, he is a thin, spider-like man with a large hooked nose and greasy black shoulder-length hair. He wears billowing black robes and pretty much dislikes everyone.
In the movies, he is Alan Rickman in a non-wizardy but completely adorable outfit.
Snape: DO NOT CALL ME THAT!
Sex in Black Robes Hogwarts Potions Master Extraordinaire, and all around snarky git. Once sworn enemy of MWPP, especially James Potter, who saved his life, and Sirius Black, who risked it. Secretly hiding a good side under all his bitching and whining. Also, inevitable true love of Harry Potter.
Characterized as a mean, slimy, suspicious and unpopular professor hated by most of the school's students, especially those outside of Slytherin.
This term can also be used by the hopeless Harry Potter geek to refer to said geek's least favorite teacher.
HP Geek #2: I know! Wait, should I be concerned that I understand what the hell you're saying when you use Harry Potter words to refer to aspects of real life?
HP Geek #1: Probably. Maybe you should go see Madam Pomfrey in the hospital wing.
HP Geek #2: Doh!