Slytherin is a house in Hogwarts. Students in the Slytherin House were considered evil because Lord Voldemort was in that house. Also, Salazar Slytherin, the founder of the house, made the Chamber of Secrets, were anyone who entered died ( except for Harry Potter!). Salazar could speak parsletongue, so the house symbol was a snake. Also because of this, the monster in the Chamber of Secrets was a basilisk.
Draco Malfoy is a commonly known Slytherin member.