Minecraft wiki entry for Player_versus_Player, associated text: The juggernaut. These are heavily enchanted soldiers. Highly costly to make!
The infantry. These are normally armored soldiers without enchantments. They are cheap on everything but Iron.
The exotic. These are users of exotic weapons (such as many potions or TNT, also including professional archers) with varying armor levels. They are not very difficult but suicidal hard in certain cases. Not very costly, but it takes time to make.