Slavery is a system under which people are treated as property to be bought and sold, and are forced to work. They can be summarily punished and even killed by their masters. The Goa'uld enslaved several species including Humans, Jaffa and Unas.
Burrock's people used the Unas as slaves on Burrock's World until the Unas rebelled. (SG1: "Beast of Burden")