Well one could say that there were just about two ways a common man could be. And then there are two means of recruiting men.
He could be a freeman, or he could be a servant. And a man could volunteer or be conscripted.
In Medieval terminology, these might be called as men at arms or sometimes mercenaries, and levies.
The serfs and peasants were subject to levy by their lords in times of war when necessary, and I imagine most non-slave classes were also. Hell, every person had some part to play in the service of war, even bishops or noblewomen.
The clergy could serve both ecclesiastic and military roles, while noblewomen paid scutage, which was money given to the duke or king, to pay for the costs of war or additional men.
Mercenaries were the landless nobles or freemen who were paid for their services in gold, rather than indentures in their service as were nobles and serfs.
So you have your volunteer mercs, and your levy conscripts lead by landed noblemen.
Of course, I may be simplifying it.
Edit: There was one more significant thing.
Retainers. Not all volunteers were mercenaries. Retainers are almost identical to the soldiers of a modern state. They had loyalty to their governors by ideology and birth, and swore oaths of loyalty. They were compensated in some way for this, and the social/economic and military roles were more intertwined than they are now.
These were men at arms. They were often less numerous than the other forms of troops though, much like you have a much larger pool of conscripts nowadays than you do of volunteer professional soldiers.