The feudal system was first established in England when William the Conqueror and the Normans came back to England after the defeat of the Anglo-Saxons at the battle of Hastings, which occurred in 1066 A.D.
At the top of the feudal pyramid was the Pope who, at the slightest pretext, could excommunicate any king, thus throwing them out of the people's favor. The kings were all afraid of this happening to them, so they tried as much as they could to be on his good side.
The second in the feudal system was the king who ruled the nation of England. He was lord over all the people in England, and commanded armies of his soldiers into battle. He also gave out all the laws and did basically whatever he wanted as long as the pope was ok with it.
Second to the kings were the nobles of the land. They lived kind of like kings, but were under his power. They were usually scattered around the country, ruling parts of it. Sometimes they would revolt and become the kings.
The fourth on the feudal pyramid are the Knights/Vassals. These men would fight for their kings when the need arises. They would not be slaves to anyone, but they would be free and have servants and slaves of their own. Sometimes they would be promoted to higher ranking positions.
The Freemen were people who were rich farmers. They lived on properties for which they paid rent to people such as dukes. They had enough money to sustain their families and not to become slaves.
Yoemen were similar to the Freemen in many ways, but were counted as halfway between servitude and being free. Some yoemen were guards, retainers (chaplains, men of arms, people who served the lords of the land and were paid for it in food, clothing, etc.), or attendants (household staff).
Servants were people who were paid for their services. They worked on other people's land and were paid by those people. Usually, they served dukes and kings. Unlikes slaves, they could leave their master's service whenever they wanted.
Peasants or serfs were the common folk who lived in small towns and villages under the dominion of the king. They usually farmed and took care of animals. Sometimes the peasants revolted because they were being abused with taxes.
Slaves were the lowest class of people in the feudal system. They were sometimes people who could not pay back debts and were taken as slaves, or they were prisoners of war. These slaves were mistreated and were never paid for their services.