Someone whose day to day existence was mostly under their own control, so long as they stayed tied to the land and produced. Both slavery and serfdom are terrible, but they are not the same.

From the perspective of industry though, serfdom is a purely agricultural institution.

Not a slave.

They couldn't be sold. They had rights. In general the classic school hierarchy of emperor, vassals, valvassori (dunno in english), valvassini (also don't know in english), serfs has been debunked.