The Pope and is church are ranked as the most important thing in society and the Feudal System.
Knights/Vassels
The king led troops to battle, had land ownership and allowed barons and lords to live on his land as long as they paid him.
Merchants/Farmers/Craftsman
Nobles swore to the king that they would fight whenever he needed them to . For that he gave the allocation land.
Peasants/Serfs
Were trusted member of nobility also those who held land as a fief and swore to protect what so ever.
A merchant would buy and sell his supplies for money, A craftsman would make the goods the merchant bought, and the Farmer would grow whet, beans, barley etc.
Serfs were bound to the land they were pretty much slaves a serfs job was a baker, farmer, blacksmith etc. and the peasants were free they could own there own businesses if they wanted to.