In Harry Potter and the Half-Blood Prince, by J.K. Rowling, a witch or wizard may use Dark Magic to divide their soul and hide it in other objects. The object then becomes a horcrux (plural: horcruxes). This must be done after an act of supreme evil e.g. murder. This grants them a form of immortality.
Voldemort can stay alive because he has seven horcruxes, including his body. He is the first person to have more than two.