Salazar Slytherin is one of the four founders of Hogwarts and the person after whom Slytherin house is named. He believed that Hogwarts should be selective about its admission of magical students, only opening doors to those who came from all-magic families. He believed that students of Muggle parentage were untrustworthy. The differences in philosophies caused a rift between the founders, and Slytherin left the school. After he departed, a legend emerged that Slytherin had left a Chamber of Secrets within the castle. The Chamber supposedly held a beast which he alone could control, which had the power to purge the school of those with less than purely magical blood. Extensive searches of the castle had not turned up any evidence of the Chamber.
During its song at the Sorting Ceremony, the Sorting Hat says that the power-hungry Salazar Slytherin valued very ambitious students over all others. Along with the other founders, Slytherin helped give the Sorting Hat brains to allow it to sort the students itself after they were gone.