[brendanm]

The 'www' is considered a subdomain, and without it is the root domain.
For example: 'www.example.com' is a subdomain of the root, 'example.com'

The benefit of using 'www' over the root is that when you get links to your site as 'www.example.com' then you can set up a separate subdomain that may not be relevant, such as a different language, like 'en.example.com'

Wikipedia is an example of a large site that uses this structure.