Slavery

Slaves harvesting sugar cane, not a lot fun for them. It is good in tea, though.

Slavery is the act of owning other humans (as well as other sapient beings by extrapolation) as property. As slaves were bound to their owners they were prevented from leaving or refusing to work under threat of immediate violence for disobedience. When two groups fought, the victor would capture some of the defeated along with goods and put them to work. Latter on as long distance trade improved they also began selling said captives. The children of slaves usually were slaves themselves. In other cases people would be put into slavery as a punishment, failure to pay their debts or voluntarily in a few cases to gain the favor of their superiors.

In addition to the practice of owning human beings as chattel there are other arrangements similar to slavery so that they are referred to as slavery informally. A few of these include Serfs (Serfs were not owned, but they were bound to the land owned by nobles and are required to work it), indentured servitude in colonial America (a guy was indentured to a contract holder and has to work for him for a term of years and the contract holder could punish said guy for failing to do work during that term), the various forced labor programs used by the Nazis and the victims of human trafficking.

Slavery in Fantasy

Slavery is one of the common features of a setting's bad guy and an easy way to establish that a certain civilization is evil is that civilization making use of slaves. A bunch of guys attacks a place with chains and whips to take catch and take away it's people so they can be taken to toil, be beaten and raped for the benefit of some bastards is more than enough reason to establish "these guys are bad so go kill their asses". However, this is not always the case; both the perceived "good" and "bad" factions can also engage in slavery, although how they do it usually defines who's good and who's bad (regardless of how minute the difference is).

Take Araby and the Dark Elves in Warhammer Fantasy's setting, for example. Both factions engage in wanton slavery and have no qualms about it being a common thing everywhere. However, what sort of defines each of them is how they see their slaves. In Araby, slaves have several rights, the slaves of children are guaranteed by law to not be slaves and particularly cruel mistreatment of slaves will result in punishment to the masters and the mistreated becoming free. The Dark Elves consider all non-dark-elves to be beneath them, and will torture and maim their slaves just because they think it is fun.

Though it is found in both slavery is more common in fantasy settings than science fiction. Why have a bunch of slaves working in a mine when you could have a bunch of robots who don't need slave drivers or food, don't plot escape/rebellion, and are stronger and easier to repair if damaged? Warhammer 40,000 actually justifies having slaves fairly well in that in the Imperium such automation is considered techno-heresy and the Dark Eldar are sick bastards who need to consume souls and get their rocks off at making others miserable.

Slavery of a certain kind is a common feature of many Magical Realms.