An indentured servant comes to work for a landowner by choice, and signs a contract for a certain number of years of labor in exchange for passage to the Americas, they do not serve for life like a slave. There was also a racial component to indentured servitude less often.
Slavery began as landowners were looking for a cheap source of labor, and slavery had upfront cost, but after that they were practically free, and could even be sold to recoup some money back. Also, Nathaniel Bacon, an indentured servant, led a rebellion in Virginia, so a source of labor that would remain docile was needed. Finally, they worked for life rather than being replaced every few years.
