In Book 6 we discover some big secrets, the biggest of which might just be that of the Horcrux. A Horcrux is a kind of magic so dark and so horrible that books about it have been removed from the Hogwarts library. A Horcrux is a kind of magic by which one can make oneself immortal by ripping one's soul into pieces and by encasing these pieces in objects. The only way one can rip one's soul into pieces is to kill another person.
Rowling uses the idea of the Horcrux to show just how horrible murder is – not only for the victim, but also the murderer. Killing someone tears the murderer's soul. We learn that Voldemort has torn his soul into seven pieces and created six Horcruxes. Through the Horcruxes we also see that Voldemort is incredibly evil. He has taken magic to a place it has probably never gone before, by making not one, but six Horcruxes. This is unheard of, and proves that he is far worse than any Dark wizard that has come before him.
Through his carefully hidden Horcruxes, Voldemort is able to flee death and to become immortal. The problem is that the more he rips his soul apart, the weaker and more decrepit his soul becomes – which explains the inhuman nature of Voldemort. However, since Voldemort doesn't value the soul, he doesn't realize that, in a way, he is weakening himself. This is another way in which Voldemort is very different from Dumbledore and Harry, who value the soul and the ability to love.
Dumbledore is in the process of discovering and destroying the six Horcruxes in Book 6. He believes that Tom Riddle's diary, which possessed Ginny Weasley in Harry Potter and the Chamber of Secrets, was once a Horcrux. He also finds and destroys the Gaunt family ring, which was also a Horcrux. Harry and Dumbledore capture the third Horcrux, the Slytherin locket, when they venture to the cave of Voldemort's childhood, but Harry later discovers that the locket is a fake, and the real Horcrux has already been stolen by someone with the initials R.A.B. That leaves three more Horcruxes and the last shard of soul which lies in Voldemort himself.
Dumbledore and Harry speculate that the three Horcruxes might be in the form of Hogwarts heirlooms, such as something owned by Helga Hufflepuff or Rowena Ravenclaw. Nagini, Voldemort's snake, may also be a Horcrux. But by the end of Book 6, all Harry has is speculation and guesswork on his side – he will have to figure out where Voldemort might have hidden his remaining four Horcruxes and what form these Horcruxes might be in. Voldemort likes to collect items with historical weight and sentimental value, trophies of his horrific acts and crimes, and so it is imperative that Harry gather all of the knowledge he has of Voldemort and his past.