Snape is the head of Slytherin House and Harry's Potions Teacher. Four times winner of the Harry Potter Most Disliked Teacher Award and the Harry Potter wooden spoon award on some websites, his favourite pasttime being making life miserable for Harry (and ron and Hermione too, for that matter).
In his youth he was the confirmed arch enemy of James Potter, Sirius Black, and Remus Lupin. He almost died in one of Black's practical jokes, but was saved by James (pity, really). He is described by the Marauder's map as a 'slimeball,' 'idiot,' a 'slimy git,' and as having an 'abnormally large nose.' He is, however, very good at making potions, and has saved Harry's life before (in Book 1).
Malfoy is, naturally, his favourite student, with Crabbe and Goyle making close seconds. He hates Sirius with a passion but is forced in Book 4 to cooperate with him by Dumbledore. Perhaps in the future...?