Serfs were the lowest class in the feudal system. In medieval Europe, they were agricultural laborers legally bound to their lord's property and obligated to perform set services for their lord. In other words they couldn't leave the lands they were on. While serfs were subject to their lord's will, they were also entitled to the lord's protection. The practice of serfdom went into decline in much of Europe at the end of medieval times.
In Russia some serfs worked as artisans and in factories. Serfdom was not abolished in Russia until 1861.
The Earth and Its Peoples A Global History, Bulliet et al, 2005.