BOOK SPOILERS

The entire story is different. For starters there's no t-rex loose in LA or whatever. Hammond is dead in the book (he gets eaten by compys at the end of the first book). There's a few details that are the same, like the fancy research lab on wheels, but in the movie the child that stows away on it is Malcolm's daughter. She's not in the book. Instead in the book two research assistants to another character are the stowaways. Small things, lots of them.

The trex getting loose is probably adapted from the first chapter in the book that predicates the entire adventure - there are reports of raptors on the mainland snatching babies out of open windows at night. I can see why Hollywood might have left that part out