Any time you do not have a weapon equipped, you will fight unarmed. Once you gain the ability to dual wield (level 10 for rogues, level 20 for warriors), any time you leave either hand unarmed, you will fight unarmed with that hand. The same goes for fist weapons. So you can have no weapon or a fist waepon in the main hand, and a big ol' sword in your off hand, if you like. Or the other way around (I hate it when games do not allow you to do this. WoW does. ^_^).
You need to pay a trainer to learn how to use fist weapons, but there is no "Fist Weapon" skill. Training in fist weapons simply allows you to equip the weapon. Fist weapons draw from and build up your unarmed skill.
By the way, single-handed weapons come in one of three types: Main Hand, Off Hand, and One Hand.
Main Hand: can only be equiped in the main hand.
Off Hand: surprise surprise, can only be equipped in the off hand.
One Hand: while usually not sporting as much damage as the other two, can be equipped in either hand.
I value one hand weapons for practice, since I can pair them with anything. But main or off hand weapons usually have higher damage per second (DPS) and neater stats/abilities. ^_^