Book: Throughout the book it is explained that Percy, after the events of the last book, has decided to side with the Ministry on the status of Lord Voldemort. Percy actually goes as far as writing a letter to Ron about choosing his friends wisely and since he is now a prefect to avoid spending time with the troublemaker and to sever ties with Mr. Potter. Also that Dumbledore is clearly not right and if anything is bothering or worrying his brother to go straight to Professor Umbridge for guidance and protection.
Movie: Percy has a few moments on screen, he does not write the letter to Ron, and there is no indication that he or the Weasley family are not seeing eye to eye.
Book: In the novel Hagrid tells Harry why he is able to see the thestrals during a lesson. In this same chapter it is also clarified that Neville can see them as well since he saw his grandfather die.
Movie: Luna tells Harry why they can see them in a complete separate scene not included in the book, in the Forbidden Forest.
Book: Sirius falls behind a curtain and never comes back. Harry waits for Sirius to get back up from behind the curtain, but when he doesn't, he is told that Sirius is dead.
Movie: Bellatrix Lestrange uses the Avada Kedavra curse on Sirius, who survives for a few moments before falling through an arch and disappearing.
Fountain at the Ministry of Magic
Book: The fountain is shown as one "monument", including the witch, wizard, centaur, and elf in one fountain. Dumbledore also brings them to life in the fight scene.
Movie: The characters are not brought to life at all, and during the short amount of time you get to see the fountains, you can notice that there are two: one for the witch, centaur, and elf and another for just the wizard himself.
Book: Ginny comes up with the title of Dumbledore's Army after Harry comments about the Ministry's belief that Dumbledore is in fact building up his own army to fight the Ministry.
Movie: The title is first shown at the top of the paper when the students first sign up at Hog's Head.
Book: Kreacher spends his days yearning for Mrs. Black, he is in love with the pureblood way of thought. And has hidden objects from the Black house in his cupboard. Including a picture of Bellatrix that he is found ogling after. He like Dobby has decided to leave his owners house and reveal the bond that Harry and Sirius have. He deceives Harry as part of the plot to lure Potter to the Ministry.
Movie:There are two scenes with this house elf without much explanation. Kreacher's obsession with Bellatrix is removed and he does not trick Harry.
Book: In the book it is explained why and how Tonks can make all her transformations. It is explained that she is a Metamorphmagus and has the ability to change apereances of her face and hair. Throughout the book she uses this skill often. Tonks appearence in the book is very weel described. She is injured in the final battle.
Movie: In the film her background is never revealed, and it is never explained why she can change form. In the film she is apparently able to change her face in to animal forms. Her role is also smaller, she only appears in a few scenes, and though she appears in the final battle, she is not injured.
Harry's Education and Aurors
Book: In the book Harry decides to become an Auror, a fact repeatedly mentioned.
Movie: This development is omitted.