They're travel guide books to Walt Disney World. The Unofficial Guide's 2007 edition is more than 800 pages long, and it's filled with useful information on how to make your trip to WDW the best it can be. Reviews of restaurants and attractions are included, as well as general advice like how to avoid foot blisters and how best to get all the most popular rides in without waiting in 2-hour lines. The tone of the book is often humorous, but it's mainly no-nonsense - their goal is to help you deal with the reality of going to a huge, very popular, and sometimes chaotic place. Lots of folks, including myself, have found the Unofficial Guide to be very useful.
The Passporter is a book that is available in two formats: spiral bound and the "deluxe" binder version. Either way, you get color maps of all the parks, in addition to reviews of restaurants, attractions, and other things to do in Disney World. The Passporter authors take a bit more of a rose-colored look at Walt Disney World than do the Unofficial Guide's authors, but they still aren't afraid to rate a restaurant or attraction poorly if it deserves it. The best thing about the Passporter in many people's opinions, though, is the Passpockets that are in the back. You get one for your travel day, plus one for each of up to 10 days at the parks (additional Passpockets can be bought if you're planning a longer trip). In each pocket you can keep your receipts and small momentos, and on each pocket there are places to write in what you did that day, funny things that happened, and so on. It makes for a great keepsake!
Like many people, I don't consider the two an either/or thing: I think both are essential. But, you can go to your local bookstore and check them out for yourself. I've seen them in both Borders and Barnes & Noble, I believe.
Happy planning!