I recently heard about this game series, and was playing the demo to the original. Warband is a stand alone expansion with an updated map and graphics, as well as multiplayer capabilities.
It's pretty much an open-world medieval action RPG with some fun gameplay. You can command an army, raid towns and villages, and even siege castles.
http://www.taleworlds.com/main.aspxI'd recommend buying Warband, as it is much better graphics-wise, plus it's pretty much the same game with multiplayer. Both are available on Steam for $29.99.