As far as I understand serfdom is very closely related to slavery. For example, Wikipedia mentions serfs could be traded, bought and sold and were more or less at the mercy of their owner/lord and even the word serf is derived from the word slave. I understand that one big distinction between serfdom and slavery, as mentioned on the Wikipedia page, is that serfs were tied to land. But it does not mention where this institution came, which is what I would like to know.
The reason why I am wondering about the above is that from my own reading on slavery I understand that it was quite heterogeneous. As explained here, slavery in ancient Rome was not the same institution as for example slavery in the US. This shows that there are already varieties of slavery, so why is serfdom considered distinct and different?
Is it just that the term serfdom is itself historic so it stuck even though there is no fundamental difference, or do historians consider these two institutions different on some fundamental level?
I also understand that slavery and serfdom were different in different countries, as mentioned in the comments, but this is what just adds to my confusion. As mentioned in the Wikipedia article, slavery is very similar to serfdom and even the word serfdom derives from the Latin word for slave, but then there are different forms of slavery and different forms of serfdom which to me implies there is some fundamental difference which is not explained in the article (or at least I did not understood it if it was).
To clarify even more, I see that the article says that there was difference in rights under feudalism between slavery and serfdom, but there was difference between rights of slaves in Rome and the US. In fact, based on the blog article, it seems to me there was bigger difference between rights of Slaves in Rome and US than between slaves and serfs under feudalism. That is what confuses me.