Hogwarts was founded over a thousand years ago by 4 of the brightest, and smartest witches and wizards of the age, Rowena Ravenclaw, Helga Hufflepuff, Godric Gryffindor and Salazar Slytherin. The four houses are named after them, and they took the students which they each liked into their house. After a while, there was a big argument between Gryffindor and Slytherin about admitting muggle-born students, and Slytherin left the school. But before he left, he constructed a hidden chamber beneath Hogwarts. Inside, he placed a Basilisk. It is called the Chamber of Secrets. When his heir returned to the school, they could release the mosnter and use it to get rid of all muggle-born students.