I've spent the last couple of days a) reading the book and b) seeing the film. I'm sure everyone has seen the Venn diagram showing the similarities between the two. Sadly, it's pretty accurate.
The book is written from the perspective of a reporter who travels around the world interviewing key survivors/leaders of the zombie war (World War Z). It is told in chronological order, from the outbreak through the panic through the planning stages and then through the defeat of the zombies. Each person's interview is sectioned into pieces, so you get multiple perspectives from places all around the globe during each timeframe.
The movie focuses on a reporter who (for some reason that isn't well explained) is God's Gift To Reporting and the military will do anything to secure his family from the ruins of Philadelphia in order to get him to use his reporting skills to help the only doctor left in the world figure out if they can make a Zombie cure. Yes, the movie acts like a mental run-on sentence. During the film he travels to South Korea, Israel and Wales in search of information. Eventually a type of cure is discovered.
The only positive thing I can say about the movie is that the zombies are really well done. The sense of growing dread at the speed and viciousness of the zombie plague is conveyed very well.
The movie missed so many moments that would have been AMAZING from the book to put on screen. The battle of Yonkers. The rise of the plague in China. The disappearance of all inhabitants of North Korea. The fortifications of Israel & Cuba (who, for all intents and purposes, "win" the Zombie war because of their locations/planning). The horrors of the undersea zombies. The nuclear strikes between Iran and Pakistan. The Chinese sub that goes rogue. The astronauts who decide to remain on the International Space Station. The tactics which eventually allow the military to effectively exterminate the zombie plague. All of these things would have been incredible on the screen. None of them were part of the movie. And, wow, Brad Pitt's kids and wife are so annoying and useless. They are the classic "reason not to be burdened down with family when the apocalypse happens" characters.
There is no vaccine in the book. Why can't Hollywood translate a compelling story to screen when one is provided to them? There HAD to be a vaccine?
I highly recommend the book. It's engaging, well written and researched, and conveys a realistic sense of what a real zombie outbreak would be like. If you like zombie effects and Brad Pitt, go ahead and see the movie. Otherwise, it is entirely missable.
For another great set of thoughts about the film, check out Felicia Day's comments about World War Z.