I started playing this game a couple years back while it was still in beta and continue to do so to this day. The concept is so simple you wonder why there aren't more games like it. Its definitely has the best replay value of any game I own. Here's some of the basics.
Combat consists of you and your army, (from 1-100+) battling another force on a rather large map. You can have many different types of horses, countless weapons and armor, and enough community mods to make the game last for years. You manually do pretty much everything, from deciding to swing from the right or left, from up high, or stab. Archery is really fun as you have to compensate for rain and gravity, lead your shots, etc.
Your character and his army gain experience as they personally get kills, and the whole group gets xp for a battle. As you gain experience, you gain levels and can raise stats like strength or intelligence, and raise skills like power draw or pathfinding. Normal soldiers level up as well and have their own progression trees but you do not control what skills they get. There are a number of special NPCs that you may hire and level up like your character however.
For 30 bucks, this game really gives you the best play per dollar than any game out there.