Slytherin values ambition, cunning, leadership, and resourcefulness; the Sorting Hat said in Harry Potter and the Philosopher’s Stone that Slytherins will do anything to get their way. The house mascot of Slytherin is the serpent, and the house colours are green and silver. Throughout the series, until the seventh book, the Head of House is Professor Severus Snape. Then, the previous Head of House Professor Horace Slughorn comes out of retirement, re-assuming authority after Snape becomes headmaster. The ghost of Slytherin house is The Bloody Baron. According to Rowling, Slytherin corresponds roughly to the element of water. The founder of this house is Salazar Slytherin.
The Slytherin dormitories and common room are reached by speaking a password to a patch of bare stone wall in the dungeons, which causes a hidden door to open. The Slytherin common room is a long, low, dungeon-style room, under the Hogwarts Lake, furnished with green lamps and carved armchairs. The room is described in the second book as having a greenish glow.
The Sorting Hat claims that blood purity is a factor in selecting Slytherins, although this is not mentioned until the fifth book. There is no reason to believe, however, that Muggle-born students are not sorted there, merely that pure-blooded students are more desirable to that house, as there are several examples of half-bloods in the house – such as Snape, Tom Riddle/Voldemort and Harry himself was only excluded from the house at his own insistence. In Deathly Hallows, a group of Snatchers claim that “not many Mudbloods” are sorted into Slytherin.
When believing Harry to be dead and thinking that he has final victory in his grasp, Voldemort proclaims his intention to abolish the other three houses and force all Hogwarts students into Slytherin. This design is foiled by his defeat and death, after which Slytherin becomes more diluted in its blood purity, no longer remaining the pure-blood bastion it once was.(All descriptions courtesy of Wikipedia)