Slytherin - one of the four houses at Hogwarts. It comes from the name of one of the four legendary founders of the School of Magic and Wizardry - Salazar Slytherin. The name is associated with something sliding or slippery (the verb slither is equal with "crawl"), or with somethin cunning. The Slytherin emblem is the snake.

Sorting Hat's songs about Slytherin:

1991: Or perhaps in Slytherin
You'll make your real friends,
Those cunning folks use any means
To achieve their ends.

1994: And power-hungry Slytherin
Loved those of great ambition.

1996: Take only those they wanted, so,
For instance, Slytherin
Took only pure-blood wizards
Of great cunning just like him.

Slytherin House tends to attract cunning, resurceful and ambitious students, mainly being pure-blood, as Salazar Slytherin put those qualities above all others. Many of Slytherins became powerful wizards and they tend to join various cliques like Death Eaters - though many of Harry Potter fans tend to villanaize Slytherin House for that, the students are all but evil. They are just ambitious and proud and those are not negative traits.

"We Slytherins are brave, yes, but not stupid. For instance, given the choice, we will always choose to save our own necks."

"You could be great, you know, it’s all here in your head, and Slytherin will help you on the way to greatness, no doubt about that."

"Albus Severus, you were named after two headmasters of Hogwarts. One of Them was Slytherin and he was probably the bravest man I ever knew."

"You happen to have many qualities Salazar Slytherin prized in his hand-picked students. His own very rare gift, Parseltongue — resourcefulness — determination — a certain disregard for rules."

Slytherin students were very intelligent and aimed for greatness. For six times during six years before the Harry Potter entered the Hogwarts, Slytherin won House Cup. And if not for additional points gained by Harry, they would win once again. That says a lot about Slytherins' hard work and fast minds - that doesn't goes only with Ravenclaws. Slytherin students also battled against the Voldemort, even if the movie wants us to believe otherwise.

Funny fact: Slytherin is first mentioned House in the books and the last. First time, Draco mentions it on Diagon Alley, the last, when Harry speaks to his son that Slytherin is a worthy House.