Domains were a mechanic in 3rd Edition that largely belonged to clerics and granted them even more additional powers keyed off of whatever type of flying spaghetti monster they chose to prostrate themselves before. 4th Edition retained a vestige of this system in the form of Channel Divinity feats that let you do other things with your channel divinity class feature, just in case you didnt have enough to keep track of.
Apparently, this wasnt enough for divine-users, and Divine Power intends to add more to the menu in the form of new Divinity feats and Domain feats. First, I'll go into a bit of detail on domains in case you missed 3rd Edition. Domains were the portfolios of a god that clued you in on what they mostly cared about. For example, Erathis is given the domains of civilization, creation, and justice. If you worship Erathis, you can take any Channel Divinity feat that requires that you worship a deity with those three domains.
This means that in addition to the Channel Divinity feat that requires the express worship of Erathis, that you can get three other Divine feats. There is, however, some overlap: Asmodeus also has the domain of civilation.
In addition there are Domain feats. There are identical to the martial feats that modify your at-will powers in various ways, excpet that they apply to divine powers. Power of Civilization gives you a +2 feat bonus to Diplomacy, and a +1 bonus to damage for each adjacent enemy when you use leading strike, mantle of the infidel, priest's shield, or valiant strike.
This sounds like a lot to take in, which is why I'm extremely pleased with the supplement distribution by Wizards. We initially got Player's Handbook with eight races and classes, and they've gradually rolled out ever-expanded options for various sources one at a time. Less rapid-rules-bloat, more time to let it all sink in before the next big thing hits. I've got a lot of Martial Power down, what with my paragon tiefling resourceful warlord/infernal strategist, and I'm still taking in the new content in Player's Handbook 2.
I'm glad they're kind enough to give us almost half a month before dropping Arcane Power in our laps.