Adding to Oliver_c answer:
It's inevitbale to change things (plot, personal stories, characters) when adaptating novels to screenplays, why? Because usually when you read a novel you are reading feelings, thoughts, internal perspectives that the characters have, those things are quite difficult to adapt to film, because in film you are seeing the character from the outside and those feelings, thoughts, etc must be portraited with actions. Unless the character narrates what he/she is feeling, you wouldn't know exactly what he/she's thinking.
It's all about action and of course screen time which can be translated to: "how much we can show in a short amount of time using XXXX money?" :P
Syd Field explains the art behind adaptation in his book "Screenplay: the foundation of screenwriting" if you are curious about it.
And from Wikipedia: http://en.wikipedia.org/wiki/Film_adaptation
Change in adaptation is essential and practically unavoidable, mandated both by the constraints of time and medium, but how much is always a balance. Some film theorists have argued that a director should be entirely unconcerned with the source, as a novel is a novel, while a film is a film, and the two works of art must be seen as separate entities. Since a transcription of a novel into film is impossible, even holding up a goal of "accuracy" is absurd. Others argue that what a film adaptation does is change to fit (literally, adapt), and the film must be accurate to either the effect (aesthetics) of a novel or the theme of the novel or the message of the novel and that the film maker must introduce changes where necessary to fit the demands of time and to maximize faithfulness along one of these axes.
Because I have read the books, I can say, yes, there have been changes in characters, situations, events, etc. But aren't too distant from the original plot / story. They maintain the principal story arch and have made a great work adaptating the story to screen.
Hope this helps a little. And forget my english grammar.