If you enjoyed the book, prepare yourself for disappointment with the movie. I was certainly not alone in this as several criticized Hollywood for the changes made to the story in the film version.
Let's begin with a few changes that did little to affect the overall story. First, the settings are different. The book is set in Rhode Island, the film is set in California. Then, there are several character changes, some more minor than others. The character of Julia Romano is not even in the film. Also, Sara (played by Cameron Diaz), is of course blonde in the movie, but a brunette in the book.
Jesse's character is somewhat different from book to film, mainly in his behavior. He is more fully developed in the book and portrayed through drug use, ongoing rebellion, and of course, multiple incidents of arson. In the film, he is seen on street corners picking up prostitutes, and left a little more one dimensional. Some critics complained that his relationship with his sister is not as developed in the movie (and I agree). Finally, Anna is only 11 in the film, rather than 13 like the book. Given the kind of decisions she is forced to make, some complained that 9 was too young to be realistic for the plot.
There is a minor plot change in the relationship between Kate and Taylor. They make love in the movie, they only kiss in the book.
The biggest (and most criticized) change in the movie, however, went unsupported by the author herself. The most surprising plot twist in the novel is of course when Anna is in the car accident and ends up dying, allowing her to save her sister with a kidney transplant after all. In the movie, Kate is the one who dies, peacefully, from her cancer, and Anna does not donate a kidney. To me, and many others, this ending ruined the movie. I cannot comment on how I would have felt if I had not read the book.