I will start out by saying that what this book is does best is it takes a completely implausible theme (zombies) and tells a story using 100% plausible personal stories as well as descriptions of economic and political fallout that you read and thing "hmm... yeah, that could actually happen". This is impressive given the basic underlying story and works on almost every level. Yes, there are a lot of unanswered questions and some of the story forces the reader to suspend a bit of belief in logic but overall the story is solid and it is a great read.
What I won't do is really talk to much else about the differences between the book and movie because they really only share the zombie war theme. I would say in some ways this book shares more in common with AMC's TV series The Walking Dead in that it tells a human survival story in the context of a zombie invasion. The World War Z movie is about Brad Pitt and zombies. Either way both the book and movie (and at least the first few seasons of The Walking Dead) are good but they do not tell the same story at all and if you have not read the book the movie doesn't spoil much (if anything) and vice versa.
Buy it on Amazon