The social structure of medieval europe is known as the feudal system. The feudal system was introduced into Europe in 1066 by William Normandy (later known as william the conqueror). He became the king after invading England and winning the battle of hastings. The idea of the feudal system was that the rulers wanted to create law and order in order to ensure the people of the land. In medieval Europe, the system was mainly used for how land would be organised. Under the feudal system, William the conqueror claimed all the land as his own. The organisation of the feudal system was the Monarch at the top ( The king ). He gives land to the nobles (lords and bishops) as payment for their military services. These nobles then give some of that land to the warrior class (knights) in return for fighting wars for the king and guarding and protecting the land of the chief tenants. At the bottom of the organisation were the peasants who paid for use of land by working for the knights and the lord's land as well as their own and paying taxes in the form of crops.
Below is a diagram of the organisation of the feudal system (also one of my references):
As we can see, the feudal system seemed pretty harsh, but as we now know, the whole idea was to keep the country fair, therefore you had to work in payment for land and well being.