The King is the one that grants the nobles (Barons, and Bishops) the fiefs. This ties in with the feudal system because not many Kings had the money to supply their own army for defence, so in order to achieve and army Kings granted fiefs to noblesmen in exchange for military tenants. The King holds the most power, and is at the top of the food chain but he relies on his land to attract noblesmen to serve him.
The barons were usually very wealthy and very powerful. They received their fiefs directly from the King and were treated very well. Allthough a baron will differentiate in rank in differnent countries across Europe, a baron tends to be very high in the social hierchy when it comes to the feudal system. Barons would need to be able to provide the King with armies of a few thousand men in order to get a fief.
A bishop was a person that held great power in the clergy in the medeival era. Bishops ruled over land called dioceses, and ruled all the churches, priests, and monestaries within the dioceses. The bishops collected 'tithes' and taxes from the people in theis dioceses. This gave the potential of a bishiop becomming very wealthy.
The lords were the people that basically ran, maintained and rented out hte land to the peasants. The lords worked over the fiefs that were given to them by nobles. These lords are also bound to be called in for military duty at anytime by their noblesmen that gave them their fiefs. A lord is a trained knight and can be called in to war to defend the King at anytime.
The peasants were at the bottom of the feudal spectrum. These people worked on the land in return for shelter and a piece of land to live on. Peasants worked and harvested crops that would feed everyone on the fief.