There are many differences between the Book and Movie. I'll be mentioning the major differences. This is part four, featuring the fifth book/movie of Harry Potter - The Order of the Phoenix.
The Order of the Phoenix
1) Room of Requirement - The room of Requirement was discovered by Neville in the movie while Dobby was the one who told about it to Harry in the Books.
2) Prefects - Ron and Hermione are prefects in the book, while this is not mentioned in the movie.
3) Quidditch - The quidditch matches, along with Ron joining as Keeper are omitted from the movie, while the book goes in to details.
4) Kreacher - When Harry calls Sirius through floo powder, Kreacher lies to Harry in the book while this doesn't happen in the movies. The way Sirius dies is the same though.
5) Betraying D.A - In the book, Marietta Edgecombe betrays the DA. It was one of my favourite parts in the book and made me laugh so much. However, in the movie, Draco and his gang catch the DA.
6) St Mungo's - Harry and his friends meet Ron's dad in the hospital after the snake bite. They accidently see Neville's parents and Professor (not really a professor) Lockhart. This whole section is omitted in the movie.
7) Firenze - When Sybil Trelawney gets fired by Dolores Umbridge, Dumbledore hires Firenze instead. While the book mentions the classes taken by him, the movies show no trace. Firenze's presence in the later books are also not shown in the movies. His only film scene is in book one.