Through murder, a dark wizard can tear apart his soul and encase it in an object known as a horcrux. Lord Voldemort divides his soul into six horcruxes which Harry and his friends must find and destroy in order to kill the last part of his soul which is contained in his body.
According to the prophecy that Sibyll Trelawney made, "the one with the power to vanquish the Dark Lord" would be "born to those who have thrice defied him, born as the seventh month dies". This prophecy could have meant either Harry or Neville Longbottom, both of who were born to parents that escaped Voldemort three times and were born at the end of July.
Throughout his sixth year, Harry recieves assistance in Potions class from notes in his old Advanced Potion-Making book which previously belonged to "the Half-Blood Prince". Later Snape reveals that he is the Half-Blood Prince, his mother being a witch named Eileen Prince, and his father a muggle named Tobias Snape.
While trying to locate Slitherin's locket at the Ministry of Magic, Harry sees "a large, round eye with a bright blue iris ...set into the wood" of Dolores Umbridge's office door, "an eye that was shockingly familiar to anybody who had known Alastor Moody".
Dumbledore is buried with the elder wand, the most powerful wand of wizardkind. Voldemort steals it from Dumbledore's grave, but since he was not the "rightful owner" of the wand it does not perform as it should. Because Snape killed Dumbledore, Voldemort must then kill Snape in order to receive the full power of the wand.