All societies are stratified, but the criteria used to categorize people vary widely. Social stratification has taken many forms throughout history, including slavery, the estate system, indentured servitude, the caste system, and the class system.
Slavery is a system of stratification in which one person owns another, as he or she would own property, and exploits the slave’s labor for economic gain. Slaves are one of the lowest categories in any stratification system, as they possess virtually no power or wealth of their own.
A common assumption about slavery is that it is generally based on racism. Though racism was the primary cause of slavery in the United States, it was not the main reason that people in other areas were enslaved. Reasons for slavery include debt, crime, war, and beliefs of inherent superiority.