|from Harry Potter Wikia|
Anyway, Salazar Slytherin was one of the founding four of Hogwarts; founding the pure-blood house of Slytherin. Nobody is exactly sure where Slytherin was born beyond "the fens" of eastern England and in the 10th century.
Slytherin was skilled at Parseltongue, Legilimacy and the Dark Arts. He wanted only pure-blood witches and wizards to be admitted to Hogwarts, and when the other founders didn't agree, he created the Chamber of Secrets and placed a basilisk inside until the time where his heir would return, open the chamber and unleash death and destruction on all muggle-borns. (ok, maybe those aren't the exact words, but look at that face. He was thinking about raining down some pain.)
So, because Slytherin House has churned out more dark wizards than any other, it has become synonymous with evil and dark magic. But my husband and a good friend were sorted into Slytherin (and I was too at first), they aren't all bad. Usually.
Slytherin House only accepts pure-blood witches and wizards. You also have to be cunning, resourceful and full of ambition. Their colors are green and silver and their symbol is the serpent. Some famous Slytherins include: Tom Riddle (Lord Voldemort), Lucius Malfoy, the entire Black family (Narcissa, Bellatrix, Regulus, et al) save Sirius, Draco Malfoy, Severus Snape, the Crabbe family, the Goyle family, many Death Eaters, interestingly enough, Merlin and obviously, Dolores Umbridge.