From the back of the book:
"A complete roleplaying game in one volume!
You've enjoyed reading about the Discworld. Now you can go there...
This book includes not only a full description of Terry Pratchett's Discworld (and, of course, its leading characters) in game terms, but also the GURPS Lite roleplaying rules. Only 32 pages long, these rules encompass everything you need to play, and are compatible with other GURPS (Generic Universal Roleplaying System) books."
Originally released as GURPS Discworld.