It's a game by Illwinter, the makers of Dominions 3 for those in the "in". In fact, CoE could be dubbed an ancestor of sorts, but it's different enough to warrant interest despite its tech being fairly outdated.
The best way to describe it, I think, would be to call it a hybrid between a roguelike and "risk". You select a character from among many different classes (basic categories are warlords, wizards, priests, and nonhuman, with subcategories in each), and start in a citadel somewhere in the map. Your starting class determines your special abilities, avaiable troops, resources that you need, etc.... Then, you hire/summon/whatever troops, and assign them to a commander and send him around the map to explore and battle for different sites, of different strategical importance (income sites, resource sites, fortresses, etc...). The goal of the game is to defeat all your enemies. And by the way, you can play hotseat coop if you like, like in the Heroes games (that would be another game to compare it with).
All in all, it's a fun little game that is very little demanding in terms of resources, can run in many different OSs, and provides fast entertainment for when you are bored.
the forum is here
http://forum.shrapnelgames.com/forumdisplay.php?f=161but the link they provide for CoEII download is broken (they restructured the webpage and forgot to relink it, apparently), but the game is still avaiable elsewhere.