I think most people are probably more familiar with the film version of The Princess Bride, starring Robin Wright and Cary Elwes (before the fathead got him). The book is--well, let's just say they cut a lot out for the movie, and that was probably a good thing.
The basic premise is that Goldman is abridging The Princess Bride from a longer version written by S. Morgenstern. This device results in pages of explanations of things that were "cut out" and why the book is abridged as it is. There are also three introductions in the version I read (the 30th anniversary edition) which go into more detail of the alleged abridging process. Of course, this is all a complete fiction, and it seemed rather unnecessary to me. I would have been perfectly happy to just have the story of Westley, Buttercup, Inigo, and Fezzik. In fact, the one thing I like more about the book than the movie was that Inigo and Fezzik had more to do, since they're my favorite characters. For those who don't know, the basic story is that of a beautiful princess and her one true love; they have to overcome a variety of obstacles with the help of a varied cast of characters in order to escape the clutches of the evil prince.
On the whole it's an okay book, but I don't think you really gain anything from the book that you don't get from the movie.