This mini-rulebook was released in a pocket edition to give players a more portable alternative to the regular Warhammer 40,000 4th Edition rulebook.
From the back cover:
Warhammer 40,000 is a tabletop game for two or more players where you command the forces of the Imperium or one of its many enemies. This book provides you with all the rules you need to play, and guides you through collecting and painting your own army of Citadel Miniatures.
Inside you will find:
BACKGROUND: Information about the Warhammer 40,000 universe and the races that populate it, their history and armies.
RULES: The mechanics for getting your armies moving, shooting and fighting over the battlefield, and a number of missions that present a stage to fight upon.