Slytherin is one of the four Houses at Hogwarts School of Witchcraft and Wizardry, and is traditionally home to students who exhibit such traits as cunning and ambition. Founded by Salazar Slytherin, the house is comprised mostly of pure-blood students, due to its founder's mistrust of Muggle-born witches and wizards. The house has a negative reputation, with many claiming that it is the source of most dark wizards in Britain, notably Tom Marvolo Riddle, a.k.a. Lord Voldemort, as well as many of his Death Eaters.
Slytherin corresponds roughly with the element of water.
Personality Slytherins tend to be ambitious, cunning, and achievement-oriented. They also have highly developed senses of self-preservation. This means that Slytherins tend to hesitate before acting, so as to weigh all possible outcomes of a decision (and how these outcomes would personally impact them), unlike Gryffindors, whose chivalrous natures would likely lead them to react immediately and instinctively.
According to Albus Dumbledore, the qualities which Salazar valued in the students he chose included cleverness, resourcefulness, determination, and "a certain disregard for the rules." Dumbledore noted that all of these were qualities possessed by Harry Potter, who was in Gryffindor.
Contrary to the wishes of the other founders of Hogwarts, Salazar Slytherin wanted a stricter policy of admission to the school —one limited strictly to pure-blood students. The controversy this created apparently led to Salazar leaving the school and the Chamber of Secrets behind. A millennium later, the house's students are still considered affiliated with Salazar's reputed views, and open expression of pro-blood purity sentiment does seem to be more common in Slytherin than in other Houses.
This disagreement over blood purity, which caused Salazar Slytherin's friendship with Godric Gryffindor to break, is the origin of the feud between Slytherin and Gryffindor, the two most competitive of the houses. Slytherin and Gryffindor are generally rivals in Quidditch and for the House Cup, which the former had won six years in a row prior to Harry Potter's arrival at Hogwarts in 1991. It is also rare for Slytherin and Gryffindor students to be friendly with one another, and common for them to share animosity. This was particularly the case during the First and Second Wizarding Wars; during the latter conflict, no Slytherin students joined Dumbledore's Army, which had Gryffindor, Ravenclaw, and Hufflepuff members, and some Slytherins declined to defend their school in the Battle of Hogwarts, with some joining the Death Eaters. However, there are exceptions, such as Slytherin Severus Snape and Gryffindor Lily Evans, who were best friends until Snape's use of the slur "Mudblood" and his aspirations to be a Death Eater caused Lily to sever their ties.