Of the Horcrux, wickedest of magical inventions, we shall not speak nor give direction ...
-- from the introduction of Magick Moste Evile, Hogwarts library (HBP18)
A Horcrux is ‘the receptacle in which a Dark wizard has hidden a fragment of his soul for the purposes of attaining immortality’ (JKR). ‘The receptacle is prepared by dark magic to become the receptacle of a fragmented piece of soul and that that piece of soul deliberately detached from the Master Soul to act as a future safeguard or anchor to life and to safeguard against death’ (PC-JKR1).
The term “Horcrux” is used to refer to any object in which a person has concealed a part of his or her soul. The object need not be inanimate; according to Dumbledore, a living creature can be used as a Horcrux, although it is risky to do so since the Horcrux in such a case is something that can move and think for itself, independently of the implanted fragment of soul.
The purpose of a Horcrux is to protect the given bit of soul from anything that might happen to the body of the person to whom the soul belongs. While the Horcrux is kept safe, the person will continue to exist even if his or her body is damaged or destroyed.
Horcruxes are indeed a matter involving very advanced, very Dark magic. Only one book in the Hogwarts library has been found to refer to them even in passing, the subject is banned at Hogwarts, and only two of the most senior professors on the Hogwarts staff are known to have any information on the topic: Albus Dumbledore and Horace Slughorn. Most of our information on Horcruxes comes from them, particularly from Professor Slughorn's memory of a fateful conversation with the young Tom Riddle and Professor Dumbledore's later analysis of that memory (HBP23).
Horcruxes were invented in ancient times, probably by the dark wizard Herpo the Foul. Herpo is also responsible for creating the first basilisk (PC-JKR1).
To create a Horcrux, by definition the spell-caster must have split his or her soul into fragments, so that one fragment can be implanted within the Horcrux while the other is retained in the spell-caster's own body. The act of splitting the soul is accomplished by committing murder, which rips the soul apart. From Dumbledore's remarks about Voldemort's possible planned use of the murders of the Potters when Harry was a baby and his assumption that Voldemort would have used the death of Frank Bryce in 1994, it does not matter whether the victim is a witch, wizard, or Muggle. According to Slughorn, some spell appears to be involved for the implant process, but Slughorn neither knew nor wished to know the details (HBP23).
Tom Marvolo Riddle, later known as Voldemort, became obsessed with avoiding death at a very young age. At some point during his early years at Hogwarts, despite the fact that the topic of Horcruxes was not part of the curriculum and was in fact banned at the school, Riddle learned of their existence.
Riddle hit upon the idea of creating not just one Horcrux to protect part of his soul, but several at once: a grand total of six Horcruxes, so that he would eventually have a seven-part soul, with six parts concealed in various Horcruxes and the seventh remaining in his body. Under a pretense of academic curiosity, he sought Horace Slughorn's opinion as to whether this would be an effective procedure, and apparently was satisfied with the answer he got (HBP23). It is worth noting that since Riddle was already in possession of his maternal grandfather's ring at the time of his conversation with Slughorn, he had at that point already committed at least four murders: those of his paternal grandparents and his father, plus the murder of Myrtle Warren.
Voldemort's Horcruxes (BLC) (full timeline)
This list is not in the order of the murders committed to create the Horcruxes. We have no way of knowing when the actual Dark Magic rituals were performed.
- Marvolo Gaunt's ring (HBP4, HBP10, HBP17, HBP23)
Death: Tom Riddle Sr., July or August 1942
Hiding place: the ruins of the Gaunts' house
A large finger ring, clumsily made out of what appeared to be gold, set with a black stone engraved with the Peverell coat of arms According to Marvolo Gaunt, the ring had been in the Gaunt family for centuries. After Marvolo's death, the ring passed to his son Morfin, but it was stolen by Marvolo's grandson Tom Riddle on the night that Tom Riddle framed Morfin for the murders of the rest of the Riddle family. Tom wore the ring openly for some time afterward at Hogwarts, but apparently after he turned it into a Horcrux he ceased wearing it. Dumbledore found the ring magically concealed in the ruins of the Gaunts' house. Dumbledore managed to "destroy" the Horcrux afterward, in the sense that it no longer functioned as a Horcrux. Dumbledore retained possession of the ring and took to wearing it for a time after its destruction, which left the stone cracked down the middle. - Diary of Tom Riddle
Death: Myrtle Warren, June 1943
Hiding place: left with Lucius Malfoy
Known to have been a Horcrux; originally an ordinary Muggle diary, but of personal significance to Tom Riddle. Left with Lucius Malfoy but subsequently planted on Ginny Weasley just before her first year at Hogwarts (CS4). Destroyed by Harry Potter by stabbing it with a basilisk fang (CS17); the remains were subsequently turned over to Lucius Malfoy (CS18). - Slytherin's locket (HBP10, HBP20, HBP23, HBP26)
Death: Muggle Tramp (BLC), possibly 1979 just before hiding it in the sea cave
Hiding place: a cave visited by Tom Riddle on a day-trip as a child
A heavy gold locket carrying Salazar Slytherin's mark (an ornate serpentine S). Inherited by Merope Gaunt Riddle, pawned during her pregnancy to Borgin and Burkes, purchased by Hepzibah Smith, and subsequently stolen by Tom Riddle upon her murder (HBP20).At some point, the locket was removed from its hiding place by Regulus Black and Kreacher, who together substituted another locket for the original and left a note for Voldemort. Kreacher subsequently tried and failed repeatedly to destroy the locket as Regulus Black had ordered him to (DH10). It was later found at number twelve, Grimmauld Place during the summer before Harry's fifth year (OP6). However, the locket was again forgotten, and later stolen by Mundungus Fletcher and sold to Dolores Umbridge. Harry, Ron, and Hermione then stole the locket while at the Ministry of Magic, and Ron destroyed it using the Sword of Gryffindor (DH13)(DH19). - Hufflepuff's cup (HBP20, HBP26)
Death: Hepzibah Smith, circa 1956
Hiding place: the Lestranges' vault at Gringotts
A small, magical golden cup with two finely wrought handles, engraved with a badger (Helga Hufflepuff's device); inherited by the Smith family from Hufflepuff. According to Hepzibah Smith, the cup in its own right possesses magical powers, but she also said that she had not thoroughly investigated the matter. Stolen by Tom Riddle after Hepzibah's murder. Destroyed by Hermione Granger with a basilisk fang (DH31). - Rowena Ravenclaw's diadem
Death: Albanian Peasant (BLC), possibly 1965
hiding place: Room of Requirement
A diadem (also called a tiara) that bestows wisdom on the wearer. It is etched with the words, Wit beyond measure is man's greatest treasure (DH29, DH31). It is destroyed by the Fiendfyre conjured by Vincent Crabbe. - Nagini
Death: Bertha Jorkins (BLC), summer 1994
A gigantic snake kept by Voldemort's side. Voldemort often used Nagini to kill people. Nagini is different from the other Horcruxes because she was not hidden away. Nagini was killed by Neville Longbottom with the Sword of Gryffindor (DH36). - Harry Potter
Death? None- Accidental and not a true Horcrux, July 31, 1981
Harry was inadvertently made into something like a Horcrux on the night that Voldemort killed his parents. When the Killing Curse rebounded on Voldemort, his remaining soul split apart, and one fragment attached itself to Harry (DH33). Voldemort destroyed this pseudo-Horcrux himself without realizing it when he attempted to kill Harry in the forest (DH36).
Commentary
Etymology
The word was created by Rowling. The word Horcrux is always capitalised in canon.
Notes
Rowling on Horcruxes:
But you know wizards would've been looking for ways to do exactly what Voldemort did for years, and some of the ways they would've tried would've killed them, so I imagine it... well, there's huge parallels. Splitting the atom would be a very good parallel in our world. Something that people imagined might be able to be done, but couldn't quite bring it off, and then... and then people started doing it with sometimes catastrophic effects. So that's how I see the Horcrux ... I would imagine that other people other people are going to have tried. I think it would be naive not to think that people have been trying for a long time, and thought they succeeded and hadn't, or else, or else you know maim themselves or kill themselves in the attempt. It's such a dangerous thing to do.
I see it as a series of things you would have to do. So you would have to perform a spell. But you would also-- I don't even know if I want to say it out loud, I know that sounds funny. But I did really think it through. There are two things that I think are too horrible, actually, to go into detail about. One of them is how Pettigrew brought Voldemort back into a rudimentary body. 'Cause I told my editor what I thought happened there, and she looked as though she was gonna vomit. And then-- and the other thing is, how you make a Horcrux. And I don't even like-- I don't know. Will it be in the Encyclopedia? I don't know if I can bring myself to, ummm... I don't know. (PC-JKR1)
Well, I tell you-- You know what, this will not end the discussion. I know that, and you know that. But here is the thing. For convenience, I had Dumbledore say to Harry, "You were the Horcrux he never meant to make." But I think, by definition, a Horcrux has to be made intentionally. So, because Voldemort never went through the grotesque process that I imagined creates a Horcrux, with Harry, it was just that he had destabilized his soul so much that it split when he was hit by the back-firing curse. And so this part of it flies off and attaches to the only living thing in the room. A part of it flees in the very close-to-death limbo state that Voldemort then goes on and exists in. I suppose it's very close to being a Horcrux. But Harry was not-- did not become an evil object. He wasn't-- he didn't have curses upon him that the other Horcruxes had. He himself was not contaminated by carrying this bit of parasitic soul. The only time he ever felt it stirring and moving was in "Order of the Phoenix", when he himself goes through a very dark time. And there's a moment where he's looking at Dumbledore and he feels something rear like a snake inside him. And of course at those times, it's because the piece of soul inside him is feeding off his emotions. He's going through a dark time and that piece of soul is enjoying it and making its presence felt. But he doesn't know what he's feeling, of course. Also I always imagined the Sorting Hat detected the presence of that piece of soul when Harry first tried it on. Because it was strongly tempted to put him in Slytherin. So that's how I see it. Now, I know that won't end the debate, but I do think that the strict definition of Horcrux, once I write The Scottish Book, will have to be given and that the definition will be: the receptacle is prepared by dark magic to become the receptacle of a fragmented piece of soul and that that piece of soul deliberately detached from the Master Soul to act as a future safeguard or anchor to life and to safeguard against death. So that doesn't clear anything up but it elucidates what I believe. But I don't think it's necessarily going to convince people who have a strong feeling, one way or the other, on the matter. You know what, that's been the case with most of "Harry Potter". I gave my explanation and it just fuels more debate (PC-JKR1).
Q: Whose murders did Voldemort use to create each of the Horcruxes?
J.K. Rowling: The diary - Moaning Myrtle. The cup - Hepzibah Smith, the previous owner. The locket - a Muggle tramp. Nagini - Bertha Jorkins (Voldemort could use a wand once he regained a rudimentary body, as long as the victim was subdued). The diadem - an Albanian peasant. The ring - Tom Riddle Sr. (BLC)