from the back cover:
All the Rules of the Game... In One Awesome Book
Tired of hauling all of your D&D rules supplements to the gaming table? Having trouble finding the rule you need? This supplement take all of the roleplaying game's most important rules and presents them in a single comprehensive, east-to-reference volume for players and Dungeon Masters.
In addition to presenting the rules of the game, this supplement incorporates official errata as well as behind-the-scenes designer and developer commentary explaining how the rules system has evolved and why certain rules work the way they do.