"AoK" is the sequel to Ensemble Studios' game Age of Empires (hence the alternate name "Age of Empires II"). Like AoE and RoR, you are in control of a civilization. Although hard to believe, the graphics are better (AoE's graphics were really good).
A major improvement is the AI - the computer players are much "smarter" now. There are also more random map types (and the random map generator is really good). For those interested in making their own scenarios, the editor now allows fancy things, like scripting, which allows neat things to happen as you play.