From the back of the book:
Any world you can imagine.
Basic Set: Campaigns combines information from Basic Set, Third Edition and Compendium II - plus our new core setting, with infinite possibilities for timeline-hopping adventure! This book and Basic Set: Characters together replace Basic Set, Compendium I, and Compendium II, plus add new rules.
This book is intended for Game Masters and advanced players. It covers physical feats, combat, vehicles and technology, animals and monsters, world design, and Game Mastering. "Tool kit" chapters let you create new creatures (and even PC races), artifacts, character abilities, and entire game worlds.
It also features a cross-world setting, Infinite Worlds, which you can use to create cross-genre campaigns . . . develop alternate histories . . . play modern characters in a fantasy world . . . whatever you like!
GURPS Fourth Edition offers regular new releases and extensive online support. It's based on 16 years of gamer feedback from the Third Edition, and is faster and easier to play than ever before. All rules are carefully organized, indexed, and cross-referenced. Charts and tables are clear and legible. And the library of 200+ Third Edition books already in print is a resource that will jump-start any campaign!
This is Book 2 of the two-volume Basic Set. Only Book 1 is necessary to play. Book 2 is required for the Game Master, and players wanting more detail will find it valuable.