Basically a domain is the web site name i.e.
www.jayne.com.
Once you have registered a domain you have several things you can do with it:
1. Set up a web site that the domain name always links to.
2. Divert it (or park it) to another domain. The idea of this is that you can register lots of different domain names that are similar to what your business does, something like
www.jayne.com,
www.jayne.co.uk,
www.jayne.eu.com - you might want to register these to stop other people getting them (and potentially stealing some of your business) or to make sure you come up high in the search engines, but when people go to any of these domain names it always comes up with your web site.
3. Lastly you might have several different areas within your business and you want to divide these into sub domains to directly access each of these different areas, but still have the main site that encompasses everything. So you have
www.jayne.com as your main site, then you have
www.jayneshop.com to take people directly to your web store and then
www.jaynerental.com and
www.jaynesales.com.
These are all ways of promoting what you do on the search engines more effectively.
Does this make more sense?
Andy
), been reading about domains. 
