A 'housecarl' literally means a 'man of the house', thus a personal retainer that is part of your 'household', with whom you share your hearth. A 'hirdman' is a 'man of the hird', or the 'army, warband', and thus a warrior that can be part of your war-group but not necessarily part of your household.

Now, the problem is that many of our terms are derived from a number of late historical sources that are in some way chosen to be representative for the entire period. The organisation of Scandinavian society in the centuries before Christianisation was rather diverse, and capturing some sort of proto-feudal organisation based on Anglosaxon terminology is a bit simple. In times of the first kingdoms, for example, a 'lid' or 'leidang' was instituted, which was quite analogous to the 'hird' levy of the Anglosaxons. The men of the Leidang did share some common 'brotherhood'-identity analogous to a household. But how far back in time does this sort of levy go? This is really still very much an unresolved matter. It would make more sense, currently, to speak of a continuum of different sorts of warriors, with a continuum of different sorts of personal, institutional, and formal bonds with other people, rather than saying there were fixed definitions of 'huscarls' or 'hirdsmen' (particularly because many sources prefer to use the Roman names too).