Well, it depends on who's using the words and when. In the original sources, it goes like this:

víkingar: Pirates, raiders, and generally unpleasantly violent people on a ship who owe no allegiance to anyone outside of their particular group

Norman: Inhabitants of northern France, after Charles the Simple gave Hrólfr Rögnvaldsson control of the territory between the Seine and Eure valleys to the sea in 911.

Northman: Anyone from the north. It's worth noting that in the Elucidarius of King Alfred, a Norwegian is referred to as a 'northman' by other Norwegians because he comes from the north of the country.

Norseman: A relatively newer term, meaning anyone who spoke a Norse language (Old Norse, Old West Norse, Old East Norse, West Norse, or East Norse) before it mutated into the contemporary languages of Scandinavia.