It can be either one.
Generally, if the manufactured home is for sale in a park, then you'll pay ground rent. However, I have heard of certain areas (Florida, for instance) where the land has been condominiumized, so that you actually do buy the land.
If the manufactured home is on its own lot, then it depends. Sometimes you buy the lot and the home. Sometimes you just buy the home, and someone else owns the lot.
So: No clear answer. If the home is listed in the MLS, there should be an answer. If it's in a park, either ask the owner (or if you'd prefer not to) or the lot manager. If it's being offered for sale privately, and it appears to be on its own lot, then all you can do is ask the seller.
Hope that helps.