Slave fiction is a sub-genre of BDSM fiction where the object of a fictional story is to show how someone adapts to the life of being someone else's slave. These stories are frequently erotic, which is part of the appeal for readers.
Some of these stories begin with the abduction of an unwilling captured subject, who is processed and acclimated to the life of a slave. In some stories, the fictional slave accepts their status as a slave after time, even revelling in it. In other stories, the slave overcomes the odds to become free again, perhaps using a combination of skill, intelligence, and strength or seduction to overcome their captors.
Some of the stories start with a well-established slavery structure and chronicle the life of one or more slaves. These scenarios are often based in either the south of the USA during the times when slavery was legal, or in some science-fiction/futuristic civilisation where slavery has some level of social acceptance.
There are stories for readers of all sexual inclinations. Many of the stories which are oriented for gays contain "straight" captured subjects whom are introduced to homosexual, especially lesbian, slavery by their captors. Heterosexual stories tend to be maledom, though a few femdom heterosexual stories are around. The most popular heterosexual stories are the Gor novels, most of which involve existing slavery situations, with some involving newly enslaved Earth women ('Captive', 'Slave Girl', 'Kajira' and 'Dancer'), with one ('Fighting Slave') involving a newly enslaved man.
The popularity of these stories is probably because they provide an acceptable way of exploring non-consensual relationships, while still leading to a 'happily ever after' ending (one way or the other...) Additionally, they provide a setting in which to play out many of the themes of Bondage, SM and D/s from real-life BDSM as a plausible part of the narrative.