Short answers, pulled from the story.
Serfdom was a condition of debt bondage and indentured servitude in which peasants worked land owned by the lord of the manor in return for protection, justice, and the right to farm certain fields for their own subsistence. It became the status of many peasants under feudalism and was tied to manorialism. It developed in late antiquity and the Early Middle Ages in Europe.
A serf could not be bought, sold, or traded individually, though in some areas he could be sold together with the land he worked, and he held certain rights in land and property. A slave, such as a Russian kholop, could be freely traded, had no rights over his own body, and could marry only with the lord's permission. Serfs were the worst fed and rewarded of the peasantry but still ranked above slaves.
A freeman usually became a serf through force or necessity, such as a local magnate's intimidation or years of crop failure, war, or brigandage that left him unable to support himself. He struck a bargain with a lord, sealed in a ceremony called bondage in which he placed his head in the lord's hands. The children born to serfs inherited their status.
Serfdom was abolished in Russia by an edict of Alexander II in 1861, freeing over 23 million privately held serfs, with state serfs emancipated in 1866. In the Habsburg monarchy it was abolished by the 1781 Serfdom Patent, in France on the 4th of August 1789, and in Prussia declared unacceptable in 1792 and abolished in 1807.
The decline of serfdom in Western Europe has sometimes been attributed to the Black Death, which reached Europe in 1347 and disrupted society by causing massive fatalities. In Central and Eastern Europe serfdom instead grew stronger, a development known as the second serfdom, becoming dominant around the 15th century and persisting until the mid-19th century.
Serfs paid their dues mainly in seasonal labour, ploughing the lord's demesne fields, harvesting crops, digging ditches, repairing fences, and often working in the manor house. They also paid taxes and fees, usually in produce, such as extra eggs at Easter or a goose at Christmas, and feudal relief when a family member died. In Poland the labour owed rose from a few days a year per household in the 13th century to six days a week per household in the 18th century.