I thought that Russian serfs were essentially slaves - a belief supported by many English-language references - but have learned that serfs are tied to land and not landowner. In the eighteenth century, Russian serfdom became de facto slavery.
During Peter The Great's reign, the size of the army was drastically increased which required a corresponding increase in supplies: metals for munitions, cloth for uniforms, grain, meat and other foods. These needs drove fiscal reform and new taxes to pay for the supplies, however, the government was unable to purchase at market price due to lack of labor. The government was forced to supply factories and landowners with cheap labor - the serfs - and within fifty years serfdom had basically evolved into slavery.