The Half-Blood Prince turns out to be Severus Snape.
Half-Blood refers to the fact that his father was a muggle (Tobias Snape) but the Prince part refers to his mother's maiden name (Eileen Prince).
Harry also learns that Fleur Delacour (from Goblet of Fire) is engaged to Ron's brother, Bill.
While the trio visit Diagon Alley to purchase robes and supplies, the meet Draco Malfoy and by an action of his (pulling his arm away), Harry guesses that Draco has been branded a Death Eater (A follower of the bad-guy Lord Voldemort).
Harry rightly suspects Draco Malfoy of being a Death Eater.
Draco manages to sneak Death Eaters into Hogwarts using a pair of vanishing cabinets.
This film is released theaters July 17, 2009 in United States Horace Slughorn: Someone else? You can only say no to these people so many times, so I never stay anywhere more than a week. Riddle, he was a quiet, albeit brilliant boy committed to becoming a first-rate wizard.
It turns out the Death Eater that overheard the prophecy and told it to Voldemort (leading to Harry's parents' deaths) was Severus Snape.
Snape turns up after Draco cannot kill Dumbledore and kills Dumbledore.
I've included a Quickie spoiler with basic important facts (not in plot order) and a long detailed spoiler that outlines the main plots.
This spoiler won't make any sense if you haven't read Order of the Phoenix or at least know some of the details.
Draco was supposed to kill Dumbledore, but finds himself unable to.