Professor Salazar Slytherin was from fen and known as the most prejudiced Founder of Hogwarts School of Witchcraft and Wizardry. He, along with his friend Godric Gryffindor and colleagues Rowena Ravenclaw and Helga Hufflepuff founded Hogwarts School of Witchcraft and Wizardry, and is the namesake of Slytherin House. He was born a pure-blood and was bestowed with the rare gift of speaking Parseltongue(which may be due to his living near a fen). He was noted as being shrewd and cunning, always determined to reach his goals and get his way. It was these traits, which he passed down to his students, that shaped his House into the supposed "dark" place it's been known as for centuries.
Despite once being great friends with Godric Gryffindor, the two broke their companionship and liking for each other after a massive argument regarding the kinds of students they felt should be allowed into Hogwarts. That is, Slytherin only wanted those of pure-blood status to be admitted because he felt muggle-borns were untrustworthy due to the persucution they were facing, something that Gryffindor and the other founders disagreed with. Not having persuaded them in the matter, he fled the school. His portrait still remains at Hogwarts.
Thought Slytherin was known to prize those of pure blood, there are records of both Muggle-borns and half-bloods being sorted into Slytherin. Slytherins are brave, yes, but not stupid. For instance, when given the choice, they will always choose to save their own necks.
The name Salazar is originally a Portuguese family name. António de Oliveira Salazar was a fascist dictator of Portugal (where Rowling lived for several years) from 1932 to 1968. The name "Salazar Slytherin" was chosen after the Portuguese dictator's name. Meanwhile, Slytherin may refer to the words "sly" as well as "slithering", which is the form of locomotion of snakes, possibly referring to his rare ability to talk to snakes, being a Parselmouth. Salazar, sometimes spelled as Salasar, is a Basque surname meaning old hall (from Castilian Sala (hall) and Basque zahar (old)). The name originates from the town of the same name: Salazar, in northern Burgos, Castile.