I'm afraid that I don't have the time at the moment to give you a fully new answer (my schedule is a bit hectic this week), but in addition to the answer linked by /u/silverappleyard, you should also find my more recent 5-part write-up about the difference between slavery and serfdom illuminating. It deals substantially with the theoretical construction of slavery and forced labour, as well as the specific differences between those two systems.

While there’s always more to say on the subject, you should check out u/sowser’s excellent answer to How comparable was slavery in the Americas to forms of slavery that preceded it?, which includes an unpacking of different forms of unfree labor.