What is the difference between serfs and tenant farmers?

A serf farms land that belongs to another, is able to retain some of his crop, and is legally bound to the land. He cannot leave for a better piece of land or a better job.

A tenant farmer farms land that belongs to another under a rental or lease arrangement. He pays the land owner for the right to farm, usually with a portion of the crop. In theory he has the right to leave and farm elsewhere, or take a job in town. In practice, tenant farmers were sometime do deeply in debt to their land owners that they could not leave because the local law considered leaving as the same as running out on the debt. Then a tenant farmer was not much different from a serf.