Reputation in Wrath of the Lich King works a lot like it did in The Burning Crusade, except refined. Most of your reputation will come from dungeon runs and questing, and not grinding mobs off in some random corner of the world. At the same time, they've thrown in some 'extra' hooks to some of the reputations that make it pretty different from what it used to be, for better or for worse. That's what we'll be digging into here.
The Horde and the Alliance each get a set of factions: The Horde Expedition and the Alliance Vanguard. These are a bit unusual, because those two primary factions have four other secondary factions. For the most part, those secondary factions don't have any rewards of their own. As you gain reputation with the sub-factions, your overall Horde Expedition or Alliance Vanguard reputation goes up, and that determines your rewards. Why did they do it that way? I don't know, but they did!