Horcrux

A Horcrux is a class of magical objects introduced in Harry Potter and the Half-Blood Prince. Its nature is central to important events in the novel, and in the Harry Potter series as a whole.

A Horcrux is a device created and used by the Darkest of wizards seeking immortal life. A Horcrux (similar to a phylactery in the role-playing game Dungeons and Dragons) is a vessel in which a Dark wizard stores his soul to prevent death. In order to do so, the wizard must essentially rend his soul into two pieces, half of the soul remaining in his body while the other half is stored in his Horcrux. The catalyst for the process of tearing the soul is committing a premeditated and malicious murder. While there do not appear to be any specific stipulations about the nature of the items that become Horcruxes, wizards commonly choose items of either sentimental or monetary value (or both) to serve the purpose, in order to prevent accidental destruction.

The importance of using a Horcrux is that while a piece of the wizard's soul exists elsewhere, he or she cannot be killed. This is exactly how Lord Voldemort escaped death during his fateful attack on Harry Potter -- he had created a Horcrux (several, as would be revealed) so that his soul was not entirely destroyed. In fact, Voldemort had created 6 Horcruxes, thus rending his soul into 7 pieces, for a part of it stayed with him. Albus Dumbledore suspected that the items that were Voldemort's Horcruxes were:
 * Marvolo Gaunt's ring, hidden in Gaunt's shack (destroyed by Dumbledore before the start of the school year in HPB)
 * Tom Riddle's diary (destroyed by Harry in COS)
 * An amulet once belonging to Salazar Slytherin, hidden in an underwater cave (destroyed or hidden by one whose initials are R.A.B.)
 * A goblet once belonging to Helga Hufflepuff
 * An unknown item belonging to either Godric Gryffindor or Rowena Ravenclaw
 * The great snake Nagini, at Voldemort's side