Unless I'm mistaken, you aren't answering his question. Tell me I'm wrong about what you are asking but here is my interpretation.
A domain is the URL that you enter such as
www.computerforum.com. That is just the URL, nothing more. When you purchase webspace, you get information from the providor. You then go to where you purchased the domain, and enter certain required information. When you get webspace from a providor without a domain, you will have a URL something like www.chem3.<providor here>.com or something like that. You go to the domain, enter the information, that way it can pull the information from your webspace, and show it on your own domain name,
www.chem3.com. I hope that makes sense.
A domain is the URL you type. Webspace is where you store the information. The domain pulls the information from the webspace to display.