Horcrux

A Horcrux is an extremely Dark magical object. Described in Magick Moste Evil as "the wickedest of all magical inventions", it is defined as any object which contains a part of someone's soul. To split one's soul, one must commit the supreme act of evil, murder. Then, by casting a spell (incantation not yet known), the murderer is able to encase a portion of their soul inside the chosen object.

The process violates two laws of nature, stating namely that Man must not kill another of his own kind, and that the soul should remain whole and intact. These violations gives the Horcrux its reputation as the most evil magical object. Horcruxes are therefore a banned subject at Hogwarts, and possibly elsewhere, too. Voldemort, obsessed with immortality, was prepared to find those few people who knew about the Horcrux and the incantation needed to create it, and then kill, to be able to immortalize himself.

Dark wizards usually choose objects of great importance to them as future Horcruxes. This might be symbolic of Muggles who are so greatly attracted to an object that after they die, they are bound to the physical world by the object, and thus cannot progress to the afterlife.

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

Known Horcruxes of Lord Voldemort

 * Tom Riddle's diary (destroyed)
 * Marvolo's Ring (destroyed)
 * Slytherin's Locket &mdash; This was most likely stolen from its hiding place by R.A.B. (who could possibly be Regulus Black), and is possibly the heavy locket which was found when the Order of the Phoenix was cleaning out number twelve, Grimmauld Place.

Possible Horcruxes (Dumbledore's theory)

 * Hufflepuff's cup
 * Nagini (Dumbledore's theory)
 * something of Ravenclaw's or something of Gryffindor's (most likely Ravenclaw's because Gryffindor's left few relics, Dumbledore owning all the known).

Horcruxes

 * Harry Potter &mdash; It is possible that Harry became Lord Voldemort's sixth and final Horcrux when Voldemort killed Lily Potter. This could explain why Harry is a Parselmouth, and why he was sometimes able to read Voldemort's thoughts. (Chamber of Secrets - Dumbledore: "Unless I'm much mistaken, he transferred some of his own powers to you the night he gave you that scar. Not something he intended to do, I'm sure ...")
 * The Sorting Hat &mdash; as an object which is a close part of Hogwarts, originally belonging to Godric Gryffindor and having a part of each house of Hogwarts in it, this is a possible Horcrux. . JK Rowling has disputed this theory on her website.
 * Tom Riddle's Hogwarts Award for Special Services to the school.
 * Voldemort's wand
 * Wormtail's silver hand

Important note: the majority of the fans see Harry or another of these instead of something of Ravenclaw's or Gryffindor's to reach the number 6. Few think there could be more than 6 horcruxes.

Locations
Known Horcrux locations are that Riddle's diary was left with Lucius Malfoy and Gaunt's Ring was left hidden magically within the ruins of the Gaunt House.

Possible locations for the other Horcruxes include
 * If the Locket is the one that Harry and the Weasleys came across in 12 Grimmuald Place, then it might be in the possession of Kreacher, it may have been stolen and sold by Mundungus Fletcher or it may still be at the Headquarters of The Order of the Phoenix. (anyway, we don't know if it's still an horcrux)
 * If Voldemort created a Horcrux when he killed Harry's father, then the Horcrux may still be in that house at Godric's Hollow. The suggestion at the end of HBP that Harry intends to visit the House in Book 7 supports this possibility.
 * The Chamber of Secrets. A location of significance to Voldemort.
 * The Orphanage. A location of significance to Voldemort.
 * The Riddle House. A location of significance to Voldemort.
 * The trophy room at Hogwarts. Voldemort likes to collect trophies and has his own trophy there.
 * In the possession of a Death Eater. This is implied in HBP, where Bellatrix states 'The Dark Lord has, in the past, entrusted me with his most precious- if Lucius hadn't-' although this may just refer to the already destroyed diary.

Murders

 * The ring: believed to have been created from Voldemort's parents murder.
 * The diary: unknow; Moaning Myrtle has been suggested, but she is very unlikely: first, a snake is not exactly a weapon, so Voldemort didn't really killed her by himself. Second, we don't know if she was a target. Third, it would be the second horcrux created, before Slughorn's interview (as Voldemort wears the ring at the interview)
 * The cup: unknow, as Hepzibah Smith is not a significant murder (or maybe she is, as Hufflepuff is her very ancester)
 * The locket: same as the cup (in all case, Hepzibah is only usable for one horcrux)
 * Nagini: Frank Brice (Dumbledore's implies it)
 * Harry Potter: James Potter
 * Voldemort's wand or Wormtail's hand: Cedric Diggory

Sites

http://www.hpfzforums.com/index.php?showtopic=4676 (where Voldemort's wand Horcrux theory came from)

Horcruxes outside of the Harry Potter universe

 * Sauron's Ring in the Lord of the Rings trilogy
 * Morda's shard of bone in Taran Wanderer