At the end of Harry Potter and the Order of the Phoenix, the reborn Voldemort finally revealed himself to the world. Harry is no longer being dogged by ill-founded claims that he is lying, making his sixth year at Hogwarts easier. Elsewhere in the magical world, Voldemort and the Death Eaters are wreaking havoc.
Dumbledore belatedly takes Harry somewhat into his confidence and reveals that Voldemort has split his soul into several pieces to assure his immortality. Only if these fragments, which are hidden in horcruxes, are all destroyed, can Voldemort be killed.
Dumbledore and Harry travel to a distant cave to retrieve one of the horcruxes. They return to Hogwarts to find it under attack by the Death Eaters. Dumbledore is killed by Snape. After the funeral, Harry vows not to return to Hogwarts, but instead to track down and destroy the remaining horcruxes.
Harry matures in this book from the whiny, angst-ridden adolescent of the previous book. His public recognition, the confidences of Dumbledore, and his developing relationship with Ginny Weasley certainly help. Dumbledore’s death after Sirius Black’s death are traumatizing events that scar Harry but leave him more determined than ever to bring down the killer of his parents.
(See the Wikipedia article for more details.)