What is a horcrux in a Harry Potter book?

Question:

What is a horcrux in a Harry Potter book?

I have read them all, but forgot!

Best Answer - Chosen by Asker: This answer contains spoilers. Please forgive my English.

Horcruxes are items to which Lord Voldemort attached a piece of his soul. To be able to do this he had to break his soul in pieces by committing a murder. Dumbledore believed there were seven horcruxes, because Voldermort seemed attracted to the number. To kill Voldemort, Harry will have to destroy all the pieces, which he doesn't know where to find, before destroying the soul-piece that is inside Voldemort's body. To this date, at least two horcruxes have been destroyed: Tom Riddle's diary and the ring that belonged to Voldemort's grandfather and uncle. It is believed that at least two horcruxes are relics the Hogwarts Founders.