Age of Empires Online trailer explains it in a nutshell
Microsoft seems to be struggling to message its first foray into free-to-play gaming, so allow this overview trailer to explain, once and for all, what Age of Empires Online actually is.
Age of Empires Online was originally developed by Robot Entertainment, but the studio wanted to pursue its own project, Orcs Must Die, so Microsoft gracefully switched in the legendary strategy veterans at Gas Powered Games.
The free-to-play social strategy game is upgradable through content packs, which are slightly cheaper if you get on board a season pass deal. Hit up the game's website for more information.
Thanks, EvilAvatar.