A serf or villien is a laborer who is bound legally to the land. They form the lowest social order of society.
Serfdom is entered into by free peasants for various reasons. Often crop failure or raids by marauding orcs or goblins will cause them to seek the protection of a lord. Serfdom is hereditary, by taking on serfdom, serfs bind not only themselves, but all future heirs.
Typically a serf rents a cottage and farmland from a landed lord and pays rents in the form of service to the lord. As part of their contract, they are expected to spend part of their time working for the lord, either on the lord’s fields or other forms of service. The rest of their time they spend working on their own fields.
Serfs are tied to the land and can not move away without the permission of their lord. In other regards, they are considered free men in the eyes of the law; they can marry and own their own property, unlike slaves.