While this may seem like a very straight forward question, it is actually a very broad topic, one which is surrounded by a lot of confusion in both the ancient sources and modern scholarship.
Firstly, there is the question of who the Helots were. It is commonly held that they were the descendants of the polity of Messene, a sort of city-state upon the slopes of Mount Ithome. Tyrtaeus tells us that it took a full nineteen years for the Spartans to conquer this territory (fr. 5 West), and that when they had conquered them, the Messenians had to provide half of all their produce (fr. 6 West). This was once taken to imply that the Messenians were enslaved following their defeat and placed into a sort of serfdom, similar to that of the Helots. However, there is no mention of slavery in the fragments, merely tribute, so while it does look promising, it isn't exactly conclusive. Furthermore, Herodotus tells us that the Helots outnumbered the Spartiates at Plataea 7:1 (Hdt. 9.28-9), and while this is a rather absurd division, it does tell us that there were a large population of Helots on hand for the Spartiates to utilise, too many to likely be the descendants of a single polity. When we consider this, along with the strategies employed by Archaic Greeks in obtaining slaves, it is likely that Helots were a heterogenous collection of nationalities. Sparta in the Classical period was known as an exclusive, inward looking society, but this reclusion only occurred in the mid-6th century BC. Prior to this, it is likely that the Spartans engaged in trade and undertook military campaigns to obtain slaves. One example of this is the Battle of the Fetters, as told by Herodotus (1.66), where the Spartans went to Tegea with the express desire of enslaving them after defeating them, so much so that each Spartan carried chains. Therefore, we can see that the Helots were likely a mixed body of slaves with origins across the Aegean, although there were some who maintained a Messenian heritage, and it was likely these who undertook the revolt of 464BC and later adopted the name Messenian (see Thucydides 1.101 and Luraghi, 2002).
If we define slavery as a complete lack of personal agency and being sold as property, then the main differences between Helotage and contemporary slavery (note: I cannot comment on Medieval serfdom with any form of authority, so I shall leave that to other more informed opinions) is that they were primarily a self-reproducing population (Thuc. 1.103 tells us that the Messenians/Helots had families), whereas other slave systems were continued via trade. It was once thought certain that Helots were tied to a particular plot of land (kleros) within Spartan territory, due to a fragment of Ephorus which tells us that Helots could not be sold "beyond the boundaries" (FgrHist 70 F117). However, there is no reason to believe that Helots were not sold between Spartiates. Xenophon tells us that Spartans shared Helots in times of necessity (Lac. Pol. 6.3), and this practice was likely a personal agreement between individuals, meaning there was no limit to the length of time that they were loaned, and by extension, it is not unimaginable that they were sold between Spartiates. However, the existence of Helot families might imply a degree of land tenure for Helots, as a permanent home is conducive to the propagation of a population, but it is likely that the Helots had no legal right to their families, as Aristotle infers with his comparison between Helots and slaves at Gortyn (Pol. 1271b41-1272a1) where a similar system was in place. Consequently, the sale of Helots, if they did occur, likely occurred with the changing of ownership of kleroi between Spartiates to encourage reproduction among the Helots, as well as the formation of knowledge regarding land that is only accumulated over several generations that would ensure the best agrarian yield (Hodkinson, 2000, p. 121). Therefore, it is likely that Helots maintained a permanent presence at particular lots, and were only sold in the most extreme circumstances, serving the owners of the plots, rather than any individual master, thus making them somewhat more secure than their contemporaries in the city of Athens (although perhaps not the countryside of Attica).
How the Spartiates monitored and managed these families of Helots when they were required to remain in Sparta as part of their civic duty, and that most of the kleroi were across the Taygetos mountains in Messenia, may seem like a mystery. However, they employed strategies common throughout Ancient Greece, having an overseer who is a slave themselves. This absenteeism is said to be the norm in Xenophon's Oeconomicus (7.29-21.12), and it is implied in Hesiod's Works and Days (van Wees in Raaflaub and van Wees, 2009). Thus, the day to day lives of Helots was likely no different from that of a slave on an Athenian or Boiotian farm.
I hope this is a somewhat satisfactory reply to you question. I apologise for not answering it completely, but as I said, I am no expert in Medieval serfdom, and I do hope that someone more well versed in that topic can add to my answer. The bibliography below should provide more than enough reading should you wish to further explore the topic.
Bibliography and Further Reading