Harry Potter and the Half-Blood Prince (movie)
Dumbledore and Harry look into Professor Slughorn's memories and find out that Slughorn taught Voldemort when he was at school. Slughorn told Voldemort about Horcruxes, objects which an evil wizard can put his soul inside of so he can be immortality. Dumbledore realises that Voldemort has been using Horcruxes to survive after death. Two of them have already been destroyed.