well those terms did exist, seeing as they were used in Roman times, but there was a difference between the two, the tax system of rome was replaced by a rent system more based on a Germanic tribal model. But then the Roman tax model disappeared from most of western europe during this time, only hanging around in italy and parts of spain.
Also, there was a difference between "unfree peasant" and slave, again coming down from Rome, though enforced by the Germanic successor kingdoms as well, the "servus" or slave and the "mancipia" being the unfree peasant. unfree peasants were like serfs, that is had their own house, provided their own food, worked the lord's land, whereas slaves were slaves, mostly didnt work the land, more body servants and household retainer types.