The three classes of people in the Middle Ages were the serfs, clergy and nobles.
During the Middle Ages, most Medieval people were peasants, over 90%, Clergy were also an important part of the social order during the Middle Ages as well as the nobility.
Serfs in the middle ages were generally peasant farmers who provided manual labor in their master's land.
The nobility was responsible with the protection of all, they had the most power of the social classes.
The clergy played an important and influential role at the time and did have a hierarchy of its own.