Okay man... well basically the whole book centers around Harry Potter growing up and becoming a bit more angry and rebellious about his parents death. He knows now that Voldemort is alive, but the ministry of magic is just stating that he's crazy and denies that voldemort ever returned. So Harry is mad, because voldemort is being spotted... etc... and they are trying to cover it up, while he feels as if he should do something about it. Furthermore, the ministry hire a teacher that is trying to dumb the kids down so that they dont get any ideas that Harry is telling the truth. In retaliation, Harry forms a group called "Dumbeldore's Army" in which he starts teaching kids defense against the dark arts, so that when the time comes, they can defend themselves. This is found out by the ministry, and chaos ensues.
Also a notable mention.. many wizards have aligned to create "The Order of the Phoenix" made up of good wizards like Dumbeldore, Snape, Professor Lupin, and Sirius Black. They have taken it upon themselves to investigate voldemorts appearance outside the ministry of magic's knowledge.
What happens at the end, is that voldemort and the death eaters invade the ministry who is completely unprepared, and Harry's group along with The Order of the Phoenix subdue the death eaters. WHile doing this Sirius Black is supposedly "Killed" (He falls behind a curtain of nothingness, so they never actually say that he died)... then you get to see the epic confrontation when Dumbeldore finally fight Voldemort one on one and damn near kills him. Voldemort manages to escape though... and the ministry finally decides to declare publicly that voldemort has returned, and they are all in some deep shit.
It also sets the stage for book 6, where Harry completes his transformation from cute little boy into a fully pissed off and ready to kill teenager
