Well, the ending was different. In the movie, Kate dies. However, in the book, Anna dies and Kate ends up getting her kidney. Kate lives on and becomes an african dance teacher.
The characters were all similar in the book, names, etc. There were a few minor differences such as [I found] Jesse's character had been butchered in the movie.
Other similarities are that Campbell Alexander is still epileptic, although he does not lie as frequently as to what his dog is for. I found the book a lot sadder, and much MUCH better than the movie. I totally recommend you read it, whether you're a guy or girl.