Horcrux

A Horcrux is an object which contains a part of someone's soul. To split one's soul, one must commit murder and then, by casting a spell, the murderer is able to encase a portion of their soul inside the object.

Albus Dumbledore and Harry Potter believe that Voldemort used as many as 6 Horcruxes to hide parts of his soul.

Known Horcruxes

 * Riddle's Diary (destroyed)
 * Marvolo's Ring (destroyed)
 * Slytherin's Locket - This was most likely stolen from it's hiding place by Regulus Black, and is probably the heavy locket which was found when the Order of the Phoenix was cleaning out Number Twelve, Grimmauld Place
 * Hufflepuff's Cup

Possible Horcruxes

 * Nagini
 * something of Ravenclaw's
 * something of Gryffindor's
 * Harry Potter - It is possible that Harry became Lord Voldemort's sixth and final Horcrux when Voldemort killed Lily and James Potter. This would explain why Harry is a parselmouth, and why he was sometimes able to read Voldemort's thoughts.