Mount & Blade: Warband is A Medieval RPG sandbox with some strategical elements and party management. It features large scale battles with troops you command, but you also fight with them. This resulting in possibly basic slaying of a couple petty bandits here and there or tracking their hideouts for lords, vassals and kings around the map (Calradia). To all out sieges on cities after betraying your king to start your own faction/kingdom. There are many possibilities!
At first, the graphics look very crap for A 2011 game but hear me out. You will invest hundreds, if not thousand of hours playing this! (But you don't need A God tier computer to run it, so that's A bonus.)
Depending on how you play, if you just want to be A bounty hunter ? Go for it. If you want to one day rule all of Calradia? You can do so however you please.
I could go on but there is just... SO much! There are a lot of things to learn when you play, but by the time you know what you're doing. It's just plain simple fun! If you finish the main game? (Which takes forever to do, and you will probably replay it at least 3 times as well.) Don't worry... There are Also mods... Lots! OF! MODS! (All the way From historical era's like French civil wars and Vikings, to Game of Thrones and Star Wars. You will never get bored.)
You will know the game once you play it because it's difficult to explain all the mechanics since there is so much going on!
Hope this gives you some idea.