"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)
Salazar Slytherin was the founder of Slytherin house and one of the co-founders of Hogwarts, described as one of “the four greatest witches and wizards of the age” (along with the other three founders) (CS9).
He believed that only pure-blood witches and wizards should be allowed to attend Hogwarts. He got into a disagreement with the other founders, especially Gryffindor, about this and eventually left the school. As well as blood purity, he also valued cunning and ambition in the students he selected for his house (PS7, GF12, OP11).
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 eradicating 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).
Artefacts: 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 and turned into a Horcrux(HBP20).
Colors: Green and silver.
Distant ancestor of Gormlaith Gaunt, Isolt Sayre, Marvolo, Morfin and Merope Gaunt and Tom Riddle.
Parselmouth and Legilimens, as well as master-builder of the Chamber of Secrets.
He may also have been an accomplished wandmaker as it seems likely he made his own wand, given that its core was made of Basilisk horn (Pm).
Other canon notes and references
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 Riddle. He is described by the Sorting Hat as "power-hungry" (GF12).
"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, which had a core of Basilisk horn, could be shut down and rendered useless through Parseltongue - a carefully kept secret through his family. His wand is now buried in the grounds of Ilvermorny and a snakewood tree has grown out of the spot where it was buried, which has leaves with powerful medicinal magic.
As Slytherin's heir, Tom Riddle did open the Chamber of Secrets 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).
Dumbledore notes that Harry has many qualities Salazar Slytherin prized in his hand-picked students: ability to speak Parseltongue, resourcefulness, determination, and a certain disregard for rules (CS18).
Named for Antonio de Oliveira Salazar, 100th Prime Minister of Portugal, a harsh authoritarian dictator who served from 1932 - 1968 according to JKR's twitter feed (JKR:Tw). 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.
From the Web
Was Salazar Slytherin a Spanish Moor? on Reddit
Salazar Slytherin by safakus