"As Rincewind involuntarily becomes a guide to the naive tourist Twoflower, they find themselves forced to flee the city of Ankh-Morpork to escape a terrible fire, and begin on a journey across the Disc. Unknown to them, their journey & fate is being decided by the Gods playing a board game the whole time."
(Above taken from this page).
They did change parts of this from the book, but - unusually for books made in to movies these days - they kept most of it accurate. Anyway, I thought it was a really good movie, and that they did a pretty good job of turning the book in to a movie. Mind you, Terry Pratchett - author of the book - was very closely involved in the work, so that is perhaps why. And how do I know he was closely involved? Well, he was so closely involved that he actually has a small role in the movie. I'm rambling a bit here though, so I'll get to the point... It's a really good movie, and I think you should try to get to watch it. Especially if you're a fan of movies containing magic, fantasy, and impossible things that could never really happen in a million years happening.