This book is the last one of the Hero Series ("complete books" for 3ed) and is a semi-sequel to Complete Divine in the same sense that Complete Mage was a semi-sequel to Complete Arcane. It concentrates on the archetype of a divine champion, giving tips on running long-term holy quest-powered campaigns, fighting for high ideals and divine intervention. Religion and holy (or unholy) power is turned into something that characters of all classes can use.
As for crunch part, Complete Champion features new feats, prestige classes, dozens of deity- and belief-themed organisations, as well as some information for Dungeon Master on designing and playing quests and holy missions. It assists the DM in helping all characters in the party to pursue divine paths simultaneously.