Traditional Medieval Society in Western Europe during the early and High Middle Ages consisted of three groups: Oratores, Belatores, and Laborores: literally, "those who pray, those who fight, and those who work." These were the clergy, the nobility, and the serfs/peasants, respectively. The Nobility were the only group allowed to carry weapons or own war horses, so they were called "those who work." A major change developed with the rise of towns and cities as serfs and peasants became craftsman. This ultimately led to the creation of the Middle Class.
A different situation developed in Eastern Europe, primarily in Poland, Lithuania, and Russia. The large population declines caused by the Black Death led Lords and Nobility to re-impose serfdom which was largely extinct in the West. The Middle Class was largely nonexistent. Whereas in the West, large numbers of people moved to the cities, free movement was forbidden in the East. In Poland, serfs could only leave the land during a two week period when they were not needed for harvest work. In Prussia, a serf who ran away was often nailed to a post by his ear, then given a knife to cut himself free.
The primary differences in demography one encounters are the differences between Eastern and Western Europe.
600 - 1450