The social classes of the Middle Ages in Europe developed as a result of the fall and or exodus of the Roman Empire post 476 A.D. The collaspe of Roman power in the west gave wat to feudalism in Europe. Feudalism was a class system which depicted a hierarchy of power beginning with the king or lord of the manor, followed by the nobles, knights, vassals, and lastly the peasants which were further broken down to freemen, serfs, and cotters.
As for jobs the hierarchy was the backbone of protection during the Middle Ages. Social status was defined by the obligation one had to the lord of the manor. Jobs ranged from the political connections of the nobles, the military prowess of the knights, the goods and services produced by the vassals otherwise known as the merchant class, and finally the masses of peasants all of who worked and farmed the land with a small distinction between those who were bound to the land and those who could leave but had no place to go, so they usually stayed.