What is Slytherin?

"The Sorting is a very important ceremony because, while you are here, your house will be something like your family within Hogwarts. You will have classes with the rest of your house, sleep in your house dormitory, and spend free time in your house common room.

The four houses are Gryffindor, Hufflepuff, Ravenclaw, and Slytherin." McGonagall, page 114, American Hardback.

This quote states that Slytherin is one of the four houses of Hogwarts. The Sorting hat also tells us that:

"...Or perhaps in Slytherin
You'll make your real friends,
Those cunning folk use any means
To achieve their ends."
The Sorting Hat, page 118, American Hardback

This statement tells us that Slytherin holds the "cunning" students. We also learn form Ron that:

"'...I suppose Ravenclaw wouldn't be too bad, but imagine if they put me in Slytherin.'

'That's the house Vol-, sorry I mean You-Know-Who was in?'

'Yeah,' said Ron" Ron and Harry, Page 106, American Hardback

So, we also know that Slytherin turned out one of the worst wizards in the wizarding world. Plus, Hagrid says:

"'Better Hufflepuff than Slytherin,' said Hagrid darkly. 'There's not a single witch or wizard who went bad that wasn't in Slytherin. You-Know-Who was one.'" Hagrid, page 80, American Hardback.

According to Hagrid, Slythering is the worst house to be sorted into.

So, What is Slytherin?
Slytherin is one of the four houses of Hogwarts in which dark witches and wizards are sorted. It is said to be the worst house.

Note: All quotes are from Harry Potter andthe Sorcerer's Stone.

I hope you now understand what Slytherin is. Enjoy the boards!