Hogwarts was founded over a thousand years ago - the precise date is uncertain - by the four greatest wizards of the age. The four school houses are named after them: Godric Gryffindor, Helga Hufflepuff, Rowena Ravenclaw, and Salazar Slytherin.
They built this castle together, far from prying Muggle eyes, for it was an age when magic was feared by common people and witches and wizards suffered much persecution. For a few years, the founders worked in harmony together, seeking out youngsters who showed signs of magic and bringing them up to the castle to be educated.
But then disagreements sprang up between them. 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.
Reliable historical sources tell us this much, but these honest facts have been obscured by the fanciful legend of the Chamber of Secrets. The story goes that Slytherin had built a hidden chamber in the castle, of which the others knew nothing. Slytherin, according to the legend, sealed the Chamber of Secrets so that none would be able to open it until his own true heir arrived at the school.
The heir alone would be able to unleash the horror within and purge the school of all who were unworthy to study magic."

- Professor Binns