While Americans typically use the term "slavery" specifically in reference to pre-Civil War slaves in the Southern United States, here's a sad but true statement: Humankind has been enslaving each other for as long as history has been recorded. The early-English enslaved Irish Catholics; the Moors enslaved the Spanish before the Spanish enslaved the Moors and Cubans; the Ottomans enslaved the Balkans and Turks; the Egyptians enslaved the Jews; the Greeks and Romans enslaved seemingly everyone. While people think of slavery as a thing of the past, it's happened in recent history (Stalin enslaved those he exiled to Siberia to work in his factories, and the Nazi labor camps are nothing but slave camps) and still happens today. In modern Africa, Nigerians still trade children and in Ghana, a man can be punished for a crime by being forced to turn over a daughter to his victim.
Historically, slaves are usually people of a different (considered inferior) ethnic, national, religious, or tribal orientation. Sometimes slaves are prisoners of war; while in other cultures, people convicted of crimes are sold into slavery. Still other times (as in the case of early-American slaves from Africa), societies plagued with poverty, overpopulation, or technological inferiority are either forcibly taken as slaves or willingly traded to more developed nations.