OK SPOILERS... dont read this post if... yea.
Basically when the book starts you find out Fudge was sacked and replaced by a guy named Scrimgeour, who used to be the head of the Auror's office and was more appropriate to lead the war on Voldemort.
Then in the next chapter, Bellatrix and Narcissa (Malfoy's mom) visit Snape at his "summer home" and we only find out that Voldemort has given Draco a difficult task that he will probably die doing, so Narcissa begs Snape to help Draco and they persuade him to make an "unbreakable vow" to help him.
Then Dumbledore goes to pick up Harry from the Dursleys, and he gets out of that house faster than he has in the previous books. They go to try and talk Slughorn, a retired teacher in hiding from Voldemort, to come out of retirement and teach at Hogwarts again. There is a surprise though... you are meant to think he will teach defense against the dark arts, but he actually teaches potions, and Snape gets the DADA class he has longed to teach.
So then the book gets underway in its usual fasion, and because Slughorn is teaching potions now, Harry is allowed to take it, despite getting an "e" on his OWL exam, allowing him to continue his persuit of being an Auror. On the first day of potions, Harry did not have his own book since he did not buy one from Diagon Alley, figuring he wouldnt need a potions book because Snape was supposed to teach. So he gets a loaner book for a few days, and this book happens to have all kinds of tricks and shortcuts and new spells written in it, done so by someone calling himself the "Half Blood Prince". Because of the helpful hints in the book, he wins a bottle of Felix Felicis, a potion that causes very good luck, on the first day of class.
Oh yea, and Harry is made Quidditch captain.. thought I'd throw that in there quick.
So throughout the book Harry suspects Malfoy is up to something, which he is, but no one really listens to Harry, thinking Malfoy isnt clever enough to do any harm. Harry is also given private lessons by Dumbledore throughout the book, each lesson a trip into a memory in the pensieve, so that Harry can learn more about Voldemorts history. His mother was a raggedy type of witch who was oppressed by her father. She used love potions on a boy named Tom Riddle to make him fall in love with her, and eventually they had a child, the baby Voldemort. She stopped using love potions thinking Tom would really love her now, but he didnt and he abandoned her to go back to his parents house. She died sometime after Vodlemort was born and he was raised in an orphanage. When he got older he sought revenge on his family and started to delve into evil.
One task set for Harry was to get a memory from Slughorn concerning Voldemort, which Slughorn would not give up easily. Harry uses some Felix Felicis to help Harry luck his way into getting the memory from Slughorn. It turns out Slughorn had given the 16 or 17 year old Tom Riddle some information about Horcruxes. Horcruxes are basically objects or animals that someone as evil as Voldemort has "stashed" a peice of their soul inside of, in order to try and remain immortal. It is finally revealed how Voldemort survived the night he attacked Harry, and it is also revealed what they must do to destory Voldemort forever. They must destory the 6 Horcruxes, and then destroy Voldemort. They have already destroyed 2, one was Riddle's diary from Book 2, and another was a ring that Dumbledore destroyed sometime before the book started, badly injuring Dumbledore's hand. And so now we know there are 4 left. Dumbledore promises Harry that Harry can come along to destroy the next Horcrux they find.
So the day comes when Dumbledore finds the location of another Horcrux, and it is not a day to be forgotten. Harry knows if Dumbledore leaves the school, it will be vulnerable to attack, so he leaves the Marauders Map and whats left of his Felix potion to Ron, Hermione, and Ginny (who he is going out with at this point) and tells them to keep an eye on the map, to watch out for Malfoy and Snape, and drink the Felix if anything bad happens.
Harry and Dumbledore go to get the Horcrux, and they succeed, but in doing so, Dumbledore is forced to drink a potion that weakens him drastically.. but he is glad to do it to destroy a peice of Voldemort's soul. Harry and Dumbledore apparate back to Hogsmeade (Harry had to do the apparating, for the first time ever, and is successful), but they find that the Dark Mark has been set on the castle. Harry and Dumbledore fly to where the Dark Mark was set, atop the Astronamy tower, and Harry has his invisibility cloak on, Dumbledore's orders. They are about to open the door into the school when they hear someone behind it. Dumbledore immediatly uses a freezing charm on Harry, in order to protect him, I guess, under the invisibility cloak. Because Dumbledore did this, it left him vulnerable to a Disarming hex that Malfoy used as he opened the door. Dumbledore is disarmed and weakened, and is at Malfoy's mercy, because Harry is still frozen.
It turns out Malfoy didnt have the guts to kill Dumbledore himself, even after several Death Eaters arrived and encouraged him to do it. A short while after, Snape comes up the stairs, which have been hexed to allow no one without a Dark Mark to pass. Snape sees the situation, sees Dumbledore in such a vulnerable position, and Malfoys inability to finish his task, and the fact that Snape is a traitor is revealed. He uses the Avada Kedavra curse on Dumbledore, and Dumbledore's body falls from the Astronomy tower (this kind of reminded me of the Mace Windu death scene, with Palpy lightning-ing him from a high building).
Because Dumbledore is now dead, Harry is unfrozen, and chases after Snape and Malfoy, trying to catch them before they can get off the Hogwarts grounds so that they can Disapparate. Harry eventually catches up to Snape, and tries to use one of the hexes he found in his Potions book. Snape says something like "How dare you try to use that hex on me, I invented it! I am the Half Blood Prince!" And the identity is revealed. Snape and Malfoy get away of course, and during the conclusion Hermione discovers that Snapes mothers maiden name was Prince, and Snapes father was a muggle... therfore Snape was a half-blood Prince (it was not a title of royalty).
So basically McGonagall becomes Headmistress (for now at least... who knows?) and Harry is deciding if he wants to go back to Hogwarts, or start a search for Voldemort's Horcruxes. By the end of the book it appears that he will not go back to Hogwarts, but I am sure he will change his mind in the beginning of the next book.
Thats about all I can think of. If I've missed any key elements, then fill in the blanks. And yes, I was VERY bored when I wrote this, and I just finished the book today, so its still fresh in my mind. I kept thinking while reading the book "ok Snape cant REALLY be a traitor to Dumbledore...." but then he goes and murders him.... oh well. Oh yea Ron finally gets snogged (kissed) by Lavender Brown... some girl I have no idea what she looks like but has been in every book so far. Surely she's no Emma Watson though... :em321:
[edit] stupid "quote in reply" button moved from the top to the bottom of the post... meh
"Guns don't kill people, I kill people."