You may have heard of divergent, but if you haven't here's a little summary; Beatrice Prior lives in the sector of abnegation but soon will choose her own path even though she is what you call divergent meeting many supporting characters along the way.
Anyways, this series is extraordinary and I personally favour the books, but what's the difference? There are actually quite a few differences when it comes to the books vs the movies.
Let's start with divergent. This is the first book in the series of three. Divergent was by far an amazing movie, but it strayed far from the book. While it may seem like it included every detail it did miss a few key points.
In the book they left out a pretty important character, Edward. He was very important to the second book and I am not sure how they are going to play that off in the second movie. He was included in a scene where a few of the guys stabbed him in the eye with a butter knife to eliminate competition. This plays a part in the second one (spoilers ahead) as he meets them again in factionless where they plot revenge on candor and overall he plays a very important role.
The movie as well neglected to use the right actresses and actors for the way the characters were supposed to look. Although Shailene Woodley is an amazing actress she did not portray tris whatsoever.
As for the second movie, Insurgent, it had not come out as a movie yet but is set to soon and it looks very unlike the book. For one Tris does not have short hair and I do not recognize a box that only divergents can open in the book but I just hope did the best.
As you can see there are few differences but that does not make them any less amazing.