A horcrux is an object inside which a witch or wizard has concealed part of his/her soul. This is done as a means to protect oneself from death, and the creation of a horcrux can only be accomplished through murder. As long as a horcrux remains intact, the person who created it can not die. Although, they will exist in a form that is less than a ghost. This is considered very dark magic. Animals and people can also be horcruxes although this is not advisable, as they are mortal. Voldemort who famously created seven horcruxes, was the first person to create multiple horcruxes.
 Destroying Horcruxes
To destroy a horcrux, one must damage the horcrux so that it can not be repaired through any magical means. Basilisk venom and Fiendfyre, also known as cursed fire, are both used to destroy some of Voldemort's horcruxes. It is presumed that Avada Kedavra, also know as the killing curse, may destroy a horcrux that is a human or animal, as the part of Voldemort's soul in Harry Potter was destroyed by this curse. However, Harry Potter was an accidental horcrux so he may be a special case. Nagini, the snake, was killed with the sword of Gryffindor, which had been exposed to basilisk venom, so it is unclear if her death alone by another means would have resulted in the destruction of the horcrux.
 Additional Cannon
 The First Horcrux
 Creating a Horcrux
To create a horcrux you have to do a series of things, including a spell. J.K. Rowling refused to talk about the details because they are too horrible. However, she said she may have to include it in the Scottish Book for it to be complete.
 The Plural of Horcrux
In the fandom horcri has been used as an alternative plural for horcrux. The use of the word horcri was made famous by the Leaky Cauldron's podcaster John Noe. This promoted J.K Rowling to confirm on her website that the correct plural of horcrux is indeed horcruxes, "as demonstrated by the eponymous chapter in 'Half-Blood Prince.'" On Pottercast 130 J.K Rowling notes that horcri was a plural she thought of, but since she already had inferi she decided against it.