The dividing line between the two is really debatable. Many people consider slavery to be essentially indefinite Indentured Servitude. But of course that means you could also argue that the best way to think of Indentured Servitude is "temporary slavery". Even more accurate would be "theoretically temporary slavery". The US's 13th Amendment may have mentioned them separately, but it outlawed both.
I say "theoretically temporary" because there's a huge incentive for the owner of the contract to ignore its end, and the two party's unequal status can provide them the opportunity to do so. This goes double if the laborer is further hampered by being illiterate, not fluent in English, and/or a different race. Escapes were common, and the typical punishment if caught was extending the term. The term could also be extended for a number of other reasons (pregnancies, illness, etc). So of course the more of those hampers you had, the easier it was to keep you on longer, or even indefinitely.
Indentured Servitude was initially by design just a way to allow poor laborers to afford to immigrate with a contract enforceable on the receiving end within English Common Law. However, the growing demand for labor meant that once the practice was established, many did not enter into their servitude themselves voluntarily. They were signed into it by parents, courts, or kidnappers. This includes white Europeans, hapless nearby Native Americans, and the first black slaves transported.