The main differences are ones that are meant to make the movie more dramatic and to highlight the main characters at the expense of lesser characters.
The best example of the first type of change is towards the end of the movie as Harry, Ron, and Hermione are trying to get to the stone. The movie leaves out the part where Hermione solves the logic puzzle. That would probably not translate that well to a movie because it is just a thought process.
The best example of the second type of change is the part with the detention in the Forbidden Forest. In the book, it is Neville who has detention with Harry, Hermione, and Draco. In the movie, however, it is Ron who has detention with them. This is presumably meant to play up the idea that these are the main characters and to strengthen our sense of their relationship.