Dumbledore tells Harry that Voldemort has six "Horcruxes," or pieces of his soul. While Voldemort has these Horcruxes, nobody can kill him. A couple of them have been destroyed. In Harry's second year, he stabbed Riddle's diary with a fang, destroying one. Before Harry's sixth year started, Dumbledore visited the old Marvolo home and found a ring and destroyed it but lost his hand as a result.