Assuming your talking about Europe. At the bottom of the system there are Peasants, the lowest and most common. They work on fields and trade and barter. Next are knights. They protect the Peasants and Nobles. They usually get paid in gold or the Peasants goods. Next are Nobles, the rich. They buy Knights to protect them and are right below the king. They usually hire Peasants to work on their land for little pay. Lastly the king. They control everything. They have knights and Nobles and oversee everyone.