Severus Snape is a fictional wizard and professor from the Harry Potter series of books and movies. During much of the series he is the Potions Master at Hogwarts school of magic, until Harry's sixth year, when he becomes the Defense Against the Dark Arts professor. Throughout the series, his position is indeterminable, and he often appears to be Harry's foe.
Background
Severus Snape was born a half-breed wizard. His mother Eileen Prince was a witch, and his father Tobias Snape, a muggle, or non-magic person. He was raised in a poor family, and made fun of by James Potter, Remus Lupin and Sirus Black. He falls in love with Lily Potter but can never have her, a fact kept a secret until after his death at the Battle of Hogwarts.
Severus Snape Timeline
The Harry Potter Lexicon: Severus Snape Timeline
January 9, 1960: Born
1969: Befriends Lily Evans
September 1, 1971: Enters Hogwarts School of Witchcraft and Wizardry | Sorts into Slytherin
June 1976: The Marauders attack him after O.W.L.S., he insults Lily | Lily breaks off their friendship
June 1978: Sits his N.E.W.T.s | Finishes Schooling at Hogwarts
1979: Becomes a Death Eater
1980: Interviews for position at Hogwarts | Overhears Sybill Trelawney's Prophecy
August 1980: Tells Albus Dumbledore about Voldemort's plans to attack the Potters
November 1981: Swears to always protect Harry and allegiance to Dumbledore
January 1996: Begins teaching Harry Occlumency
April 1996: Harry discovers worst memory | Occlumency lessons end
June 1996: Alerts the Order of the Phoenix to the Battle of Department of Mysteries
Summer 1996: Swears to kill Dumbledore for Draco Malfoy
September 1996: Takes over Defense Against the Dark Arts Position
June 1997: Kills Dumbledore
August 1997: Made Headmaster of Hogwarts by Death Eater Administration
December 26, 1997: Hides the Sword of Gryffindor in the Forest of Dean so Harry can find it
May 1998: Gives his memories to Harry before death | Dies