Long ago, as you know, Hogwarts was founded by the four greatest witches and wizards of the age, Helga Hufflepuff, Rowena Ravenclaw, Godric Gryffindor, and Salazar Slytherin. Now, most of the founders were pleased with the school they had created, but one was not. Salazar Slytherin believed they should only teach children of wizard descent, that anyone with Muggle blood was unworthy to be taught at the school. The other founders disagreed, so Slytherin left the school. 

He created a secret chamber before he left, in which only the Heir of Slytherin could open and unleash the monster within on any student he believed unworthy to study magic. For nearly a thousand years, this Chamber became mere more than a legend. The school had been searched many times, but no one could find any evidence of the Chamber, not even the Head of Slytherin House. 

But when Tom Riddle (Voldemort) arrived at school he heard the monster within the Chamber speaking to him whenever he approached near the girl's bathroom on the second floor. When he was sixteen, he opened the Chamber of Secrets, determined to know what was speaking to him.

He released the monster, and merely a day later, a girl was killed in that same bathroom. Voldemort knew he needed to contain the basilisk before it could kill anyone else and get him expelled. So he blamed a third year named Rubeus Hagrid, who was expelled although fully innocent. 

Tom Riddle left behind a diary so that in years time, another could open the Chamber and finish what he had started. Fifty years later, Lucius Malfoy, a loyal Death-Eater, placed the diary in Ginerva Weasley's cauldron. The diary possessed her as she poured her soul into it, and she opened the chamber. No one was killed, although several were petrified for weeks and even months. Harry Potter saved Ginerva Weasley from Voldemort, and slayed the basilisk.