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!
Harry: Oh My!
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!