Serfs provided most of the labor in feudal society. Most European countries ended the practice of serfdom in the Middle Ages, but Russia retained the practice until February 19, 1861. Parts of Europe, including much of Scandinavia, never adopted feudal instutions, including serfdom.
The word serf re-appered in the late 20th century to refer to a wage slave working in a capitalist business enterprise. Note one memorable coinage: the noun Microserf, which refers to employees of Microsoft Corporation, with the connotation that they become tied to that corporation, instead of the land, and work long and hard for the benefit of their masters. (See for example the novel Microserfs by Douglas Coupland.) When owners sell companies, modern serfs (or at least the jobs they perform) may get sold along with the companies for which they work.