Let's just say in the year 1820 to put a timeframe on the question.
After having learned about both American slavery and Russian serfdom this year, some details about the two institutions struck me as remarkably similar. What I am wondering is whether serfdom and slavery really were so similar, or whether the similarities are more superficial.
The biggest difference is that serfs are legally tied to the land. If you were a boyar and you bought an estate, the serfs came with it. You could not sell a serf to another estate. Serfs also had legal families, a privilege slaves were not entitled to.
One of the biggest differences was Emancipation. While much is made of Alexander II's 1862 Emancipation of the Serfs, the law expected serfs to compensate their former lords for their freedom...and if they couldn't pay they could be forced into up to 50 years of continued labor until they paid off their "debt." The last of these obligations were cancelled in 1906, 44 years into the intended 50 years, so you could easily say that serfdom didn't truly disappear until the 20th century.
Sheila Fitzpatrick's The Russian Revolution discusses this delayed end to serfdom.