The term horcrux is an invention of J.K. Rowling, first mentioned in the sixth Harry Potter novel, Harry Potter and the Half-Blood Prince. Creating a horcrux occurs when one rips the soul by committing an intensely evil act, such as murder. The portion of the soul ripped away is then encased in some type of meaningful object. Thus, if the physical body of a person with a horcrux were destroyed, the person would still not be dead, unless horcrux containing the piece of soul was also found and destroyed.
Rowling uses the term to explain why Voldemort managed to survive a backfired killing curse when he attempted to kill the one-year-old Harry. Much of the sixth book deals with Headmaster Dumbledore instructing Harry on the horcruxes that Voldemort likely created. These horcruxes kept Voldemort alive, though lacking in a body until the end of Book Four, Harry Potter and the Goblet of Fire.
Through the sixth book, Harry learns that of all places most dear to Voldemort, Hogwarts, the school of witchcraft and wizardry, was the most so. Thus, objects from the school founders are likely places for Voldemort to conceal horcruxes. Dumbledore also suggests that Voldemort created not one horcrux, but more likely seven. As a result, Harry’s final goal is not only to vanquish Voldemort, but to destroy the remaining horcruxes as well.
So far, two horcruxes have been removed. Harry destroyed one in the second novel, Harry Potter and the Chamber of Secrets, in the form of a diary that Tom Riddle/Voldemort made when he was about 16. Dumbledore destroys another at the beginning of the sixth novel. This suggests five more horcruxes remain to be destroyed.
From Book Six, we know that a locket belonging to Salazar Slytherin, one of the founders of Hogwarts contains a horcrux, though it is not hidden where Voldemort thinks. A fake horcrux contains a note from R.A.B. saying he has taken the real horcrux. R.A.B. is probably Regulus Black, and the locket is probably in a cabinet at the house Harry inherits from Sirius Black, Regulus’ brother and Harry’s godfather.
One other horcrux is probably contained in a cup belonging to Helga Hufflepuff, another founder of Hogwarts. Possibly, Voldemort’s snake, Nagini, contains another. This leaves two more to be found, possibly in objects owned by the other two Hogwarts founders, Ravenclaw and Gryffindor. Some have also suggested that Harry himself, as an intended victim of Voldemort's crimes, may be a horcrux, possibly leading to Harry's own death as a part of Voldemort's defeat.
By creating a horcrux and tearing the soul, the wizard becomes less human. Creating one horcrux is bad enough. Creating seven is horrendous in the Harry Potter world, since the person becomes increasingly less human, and operates with a tiny disfigured soul. When Dumbledore gives this knowledge to Harry, he suggests that because Voldemort has only a bit of his soul left, Harry has some advantage. Harry fights with a pure and complete soul, thus possessing power that Voldemort “knows not,” as Book Five suggests.
There are some theories on online Harry Potter sites, however, that Dumbledore may have created a horcrux himself, however, when he defeated the evil wizard Grindenwald. Most suggest the horcrux may be located in Dumbledore’s faithful phoenix, Fawkes. This is questionable at best, but may suggest that Dumbledore is not actually dead.
However, Dumbledore has always suggested that the natural progress of life leads to death, and refers to it as “the next great adventure,” at the end of Book One. It seems unlikely Dumbledore would tear his soul in this fashion since he values the whole and pure soul above all else.
What can be said of the horcrux is that it is metaphoric for the destruction that occurs to the soul when one commits an evil act and fails to repent in any fashion. The remaining horcruxes are likely to figure largely in the final book of the Harry Potter series, since they are the key to completely defeating Voldemort.