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.