What's the Armory, you wonder? Well, it's a new part of the official Blizzard site (currently in beta testing) that offers access to character profiles (for characters above level 10), guild profiles, arena team profiles, and arena ladders via the web. The nice thing about Blizzard doing this is that it's all integrated -- no extra software to install, no profiles to keep up to date -- Blizzard already has all of the information and has just given us a new way to access it. The only problem: the Armory website doesn't seem to be online right now. No doubt the cause of millions of players simultaneously trying to hit the site and see how it works.
Update: European players have their own version of the Armory, and it seems to actually function right now.