Basically a sandbox, kind of like Pirates, I like the concept of not being just some storyline you have to follow to advance the game. It is very much and entirely free play. You do have the availability to do some quests and those can effect your reputation but there is no real winning exactly. I think you could eventually become the ruler of the entire land if you choose that route.
I love Mount and Blade. Warband is the best version of it, in my opinion. You advance by gaining XP, which is done by doing quests for various people (collect taxes, kill bandits, kill a wanted man, deliver a letter, etc.) and by defeating enemies (killing them or knocking them unconscious) and winning battles. You build your own warband, and level them up as they gain XP as well. It's hard at the start, but once you level up enough and get good equipment, you can become very strong. Just remember you're never invincible: you always need an army with you to fight by your side, or you will be whittled down and knocked unconscious–no matter how high level you are and how good your armor is.
A good way to earn money is to take prisoners and ransom them. There are NPCs in taverns you can ransom them to. They are not in every tavern, but travel from city to city.
Once you gain enough reputation by fighting and winning and doing quests, you can pledge your sword to a faction. Or you can (if you dare) attack a fortification and take it for yourself, creating your own faction.