In the feudal system, there were lords (the owners of land and the offerers of protection); vassals (those who fought for the lords in exchange for tracts of land or payment); and serfs (those who worked the land in exchange for protection).

In the feudal system, the land was worked by the serfs, who essentially entered into a state of voluntary slavery in exchange for protection from invasion. The lords offered protection and the vassals were paid to carry out the protection. Of course, only the first generation of serfs "volunteered" for the slavery; later generations were stuck working as effective slaves even once the threat of invasion was lessened, hence the longevity of feudalism in Europe, which in some places lasted for as long as a thousand years.

Feudalism arose in Europe during the so-called "Dark Ages" in response to the power vacuum left behind by the Roman Empire. The institution was then strengthened in the ninth and tenth centuries after the fall of the Carolingian Empire. The people of Western and Central Europe were being relentlessly invaded and plundered by the Vikings, the Magyars, the people of the Asiatic Steppe, and Muslim Caliphates. Without a strong king to protect them, the people turned to local landowners, who established a system whereby the lords would provide protection in exchange for the common people working their lands. Labor for protection is the underlying tenet of feudalism.

Feudalism was the prevailing economic and social system throughout much of Europe during the Middle Ages. Under the Feudal System a landowner would grant a "fief" or "fiefdom" to a vassal who in exchange would pledge "fealty" or allegiance to the landowner and agree to fight to defend his lands.

"Villeins" were peasant farmers, or serfs, in Feudalism. They were tied to the land and many of their rights were directly granted and controlled by the nobleman who owned the land. They were distinct from slaves, who had no political rights of their own, and freemen, who were free to leave the land and seek work elsewhere. 

The Feudal System was introduced to England following the Norman Conquest in 1066. The Norman Conquest is the name given to the invasion of William the Conqueror, also called William Duke of Normandy, who invaded from modern-day Northern France and defeated the Anglo-Saxon King of England Harold of Wessex. 

A "motte-and-bailey" is a type of early Feudal castle. The Feudal System emerged in response to the chaos of the Dark Ages. It relied upon a landowner giving grants of land to knights - in exchange for loyalty and protection of his land. And rights to work given to serfs - in exchange for the assurance of their safety and the ownership of their labor. The system necessarily relied upon defensive fortifications that could protect small communities. A "motte-and-bailey" castle was a type of early castle, where the "motte" was a raised-earth stone or wood castle and the "bailey" was an accompanying enclosed area surrounded by a palisade.