A) as a customer you have one account with many domains in that account, basically keeping all your domains under one user name and password.
b) you own blue-widget.com red-widget.com and yellow-widget.com all these site point to to website or part of your website.
c) you have one hosting account say 100meg space with 5gb transfer a month. you then can set up folders for different domains in that one hosting package keeping all you stats and bandwidth charges to a minimum
The real answer I suppose is that ever the hosting company want to offer under that description.
DaveN