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.
A device created and used by the darkest of Dark witches and wizards, those seeking immortality. In a horcrux, one conceals part of the soul to prevent death. In order to do so, the soul must be torn into two pieces: one for the horcrux, and one that remains in the physical body. To create a horcrux, one must commit "the supreme act of evil" — murder, which "rips the soul apart." They must then perform a special spell (the incantation of which has not been revealed) which will harness their soul, allowing them to place it into an object at will.
The act of creating a horcrux is considered to be dangerously unnatural.
He-Who-Must-Not-Be-Named has several horcruxes that are hidden; and only when they are all located can the Dark Lord be defeated.
A bit of rare dark magic, in which one takes a bit of their soul and hides it in an object. The soul is split by committing the greatest evil there is- murder. Very dangerous, very tricky, best not to try it unless you're willing to end up looking like Voldemort.
1.
Tom Riddle: "I still don't quite understand how that works, though, sir."
Slughorn: "Well, you split your soul, you see, and hide part of it in an object outside the body. Then, even if one's body is attacked or destroyed, one cannot die, for part of the soul remains earthbound and undamaged. But of course, existence in such a form... few would want it, Tom, very few. Death would be preferable."
-Harry Potter and the Half-Blood Prince by J.K. Rowling
2.
Bellatrix: "No, there are pieces of you missing!"
Voldemort: "Are you talking about my horcruxes?! Because if it wasn't for them I wouldn't even BE here right now!"
-A Very Potter Musical