Snape is generally described as hook-nosed, greasy-haired, pale and thin. His demeanor towards Harry Potter is closest to loathing, for reasons unbeknownst to Harry, until the 7th Harry Potter book. In the final installment, Harry discovers that Snape loved his mother Lily. Snape also despised Harry's father, James, perhaps because of jealousy.
Although Snape hates Harry due to Harry's resemblance of James Potter, he sees his own love's eyes in Harry. Snape is largely responsible for saving Harry's life throughout his school career by working as a double agent for Dumbledore. His death is quick and without glory, but Harry spreads the knowledge of Snape's bravery in the years after his death.
Dumbledore,with tears in his eyes: All this time?
Severus Snape: Always.
The Potions Master at Hogwarts.
Likes: (Defense Against the) Dark Arts, Lily Evans, Potions, Lily Evans, Scaring first-years, and Lily Evans
Dislikes: James Potter, Harry Potter, Voldemort, and a lot of other things...
He's also quite a good runner. Nobody runs faster than "Snape confronted with shampoo."
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.