"A rift began to grow between Slytherin and the others. Slytherin wished to be more selective about the students admitted to Hogwarts. He believed that magical learning should be kept within all-magic families. He disliked taking students of Muggle parentage, believing them to be untrustworthy. After a while, there was a serious argument on the subject between Slytherin and Gryffindor, and Slytherin left the school."
-- Professor Binns (CS9)
One of the original founders of Hogwarts “over a thousand years ago.” Salazar Slytherin believed that only pure-blood witches and wizards should be allowed to attend Hogwarts. He got into an argument with Godric Gryffindor about this and eventually left the school. There was a legend that Slytherin built a secret chamber somewhere in Hogwarts that only his true heir would be able to open. This chamber, called the Chamber of Secrets, contained a monster that would finish his “noble purpose” of killing all the Muggle-born students at Hogwarts (Pm). According to legend, Slytherin reconciled with the other Founders later in life and came back to Hogwarts castle to die (JKR-W3).
As Slytherin’s heir, Tom Riddle did open the Chamber in the 1940s and released the monster, a basilisk. In 1993, Harry Potter, a Gryffindor champion, defeated the basilisk with the help of Fawkes the phoenix and the sword of Gryffindor. In many ways, this battle was the culmination of the animosity between Salazar Slytherin and Godric Gryffindor which began a millennium before (CS17).
Artifacts: A heavy gold locket carrying Salazar Slytherin's mark (an ornate serpentine "S"). The locket was inherited by Merope Gaunt Riddle, pawned during her pregnancy to Borgin and Burkes, purchased by Hepzibah Smith, and subsequently stolen by Tom Riddle upon Smith's murder (HBP20).
Colors: Green and silver.
Dumbledore notes that Harry Potter has many qualities Salazar Slytherin prized in his hand-picked students: ability to speak Parseltongue, resourcefulness, determination, and a certain disregard for rules.
Parselmouth and Legilimens, as well as master-builder of the Chamber of Secrets
Other canon notes and references
Salazar Slytherin was something of an egomaniac, which is why he built the Chamber of Secrets as a sort of monument to himself, as explained on Pottermore. This also may explain his appeal to another wizard who was impressed with himself - Tom Marvolo Riddle.
"Perhaps, when he first constructed the Chamber, Slytherin wanted no more than a place in which to instruct his students in spells of which the other three founders may have disapproved (disagreements sprung up early around the teaching of the Dark Arts). However, it is dear by the very decoration of the Chamber that by the time Slytherin finished it he had developed grandiose ideas of his own importance to the school. No other founder left behind them a gigantic statue of themselves or draped the school in emblems of their own personal powers (the snakes carved around the Chamber of Secrets being a reference to Slytherin’s powers as a Parselmouth)" (Pm).
Salazar Slytherin's wand can be shut down and rendered useless through parseltongue, a careful kept secret through his family. His wand is now buried in North America and a snakewood tree has grown out of the spot where it was buried and has leaves that have medicinal uses.
Possibly named for Antonio de Oliveira Salazar, 100th Prime Minister of Portugal, a harsh authoritarian dictator who served from 1932 - 1968. J.K. Rowling lived in Portugal for some time and her first husband was Portuguese.
Based upon the image by Lightmaker from JKR's website (shown here), Slytherin is gaunt with a long grey beard and pale grey eyes. Canon, or artistic license? Only Jo (and Lightmaker) know for sure.
JKR's "Wizard of the Month" for June 2007.
In the Chamber of Secrets film, Harry literally stands on the head of the Slytherin statue as he slays the Basilisk. In the books, the statue includes the body and legs of Slytherin, but Harry never climbs up on it.
"Slytherin from Fen"
As mentioned above, the Sorting Hat tolds us that Salazar hailed from "fen" or the marshy areas of eastern England. Mugglenet Alohomora! podcast commentors discussed the associations of marshes with creatures such as snakes and other reptiles which go hand and hand with Slytherin's snake-like character.