Serf

A Serf is an indentured servant who does not own the land he works. They are the lowest rank of the third estate.
Usually they have to provide their liege with a tenth of all their produce in exchange for being allowed to work the land. Depending on the local laws and customs, a serf might actually be able to buy the land he is working off his liege or purchase a different plot of land and become a Burgher that way, yet there are also cases of serfs being bound to their lord for their entire life.