In the Harry Potter
series, one of the houses at Hogwarts School of Witchcraft and Wizardry
Salazar Slytherin was one of the four original founders of the school. He hand-picked his own students by certain criteria, and they stayed in Slytherin house, where they stay together like family.
Professor Albus Dumbledore said that the qualities Slytherin prized in his hand-picked students were Parseltongue (The ability to communicate with snakes), resourcefulness, determination, and a certain disregard for rules. Unfortunately, not many people seem to like Slytherin, for many reasons.
For a few years, the founders worked in harmony together, seeking out youngsters who showed signs of magic and bringing them to the castle to be educated. But then disagreements sprung up between them, a rift began to grow between Slytherin and the others. Slytherin wanted to be more selective about the students admitted to Hogwarts. He believed that magical learning should be kept within all-magic families, so called Pure-bloods. He disliked taking students of Muggle parentage, believing them to be untrustworthy. After a while, there was a serious arguement between Gryffindor and Slytherin and Slytherin left.
Before he departed, he created a Chamber of Secrets with a Basilisk within to kill the Muggles. Only an heir could open the sealed chamber.
The symbol for Slyherin was the snake, and the colors are white and green. They won the Quidditch cup for years until Harry Potter helped the Gryffindor team win as seeker.
The sorting hat every year describes them:
Or perhaps in Slytherin
you'll make your real friends,
Those cunning folk use any means
To achieve their ends.
--From Harry Potter and the Sorcerer's Stone / Philosopher's Stone.
And power-hungry Slytherin
Loved those of great ambition
--From Harry Potter and the Goblet of Fire
Harry was almost placed in Slytherin house, but chose Gryffindor. The best description to date all of Slytherin house is in Harry Potter and the Chamber of Secrets.
Harry didn't want Slytherin because nearly every wizard and witch who turned to the dark side with Lord Voldemort came from Slytherin. The reason is simple, but overlooked by many readers. Voldemort's aide Quirrell said "There is no good and evil, ther is only power, and those too weak to seek it..." All Slytherin's want power, and many turn to the dark side to obtain it.
Most people in the Harry Potter fan clubs don't want to be Slytherin, partly because all of Harry's enemies are from there, like Draco Malfoy, Lord Voldemort, Professor Snape... There are some who like Slytherin for the shrewdness of the people within. (see the 2nd writeup for Gryffindor.)
The best description to date all of Slytherin house is in Harry Potter and the Chamber of Secrets. The last quote is from the first book, Harry Potter and the Sorcerer's/Philosopher's stone.