Three primary elements characterized feudalism: lords, vassals and fiefs; the structure of feudalism can be seen in how these three elements fit together. A lord was a noble who owned land, a vassal was a person who was granted possession of the land by the lord, and the land was known as a fief. In exchange for the fief, the vassal would provide military service to the lord. The obligations and relations between lord, vassal and fief form the basis of feudalism.
http://en.wikipedia.org/wiki/Feudal_system