As far as I can remember there isn't a Rulebook definition which sets them apart. Railway nomenclature varies very much with region and era. Whenever a question like this comes up there tends to be a multitude of answers, many of which appear to contradict each other but are actually correct in their own context.
Are you talking about the present, or do you have a specific time/place in mind ?
(My particular interest is Bricklayers Arms depot, which had Willow Walk sorting sidings associated with it. They certainly marshalled freight and passenger trains there, but never referred to it as a marshalling yard.
)