At the end of the civil war, both Indentured Servitude and Slavery where abolished in the United Stated.
The earliest Africans imported into the colonies, who weren't actually directly imported from Africa, but usually captured from Spanish slave ships, were imported as Indentured Servants.
Indentured Servants where bound to work for a certain party for a certain fixed period. After that period, they where free to do whatever they wanted.
Possibly, captured slaves where given Indentured Servant status to alleviate the moral implications of slavery. For the Africans themselves, being servants was better than being slaves as well, since after a number of years, they would be free to do as they please.
Indentured Servitude also wasn't limited to Africans. For many white people, it was a form of paying of a loan, when you had no collateral or income. Many immigrants either already arrived in the new world as indentured servants, their master having paid for the transfer to the new world, or became indentured servants, upon finding themselves penniless in New York. In exchange for Indentured Servitude, they obtained transport to the frontier, room and board during a number of years in exchange for work during that period, and afterwards, they would receive a plot of land to start their own lives on.
It is assumed that slavery developed from indentured servitude, possibly as a form of punishment, or as a form of gradually lengthening the period untill the people where effectively slaves.
Indentured Servitude however, was not hereditary, hereditary Slaver was only established by law in the Colony of Virginia in 1662
Indentured servants could be punished for running away, and a common punishment would be to add time to the contract. At least, this was the common punishment for white people. For black people it was usually physical punishment, indicating that it was the idea of having them as servants for life anyhow.
At the end of the civil war, both Indentured Servitude and Slavery where made illegal, in part because Indentured Servitude had become a way to get around slavery being illegal in some regions. Just have a "free man" "voluntarily" sign an indentured servitude contract of 99 years, and you have a slave in effect, without officially falling under slavery.