Domain name does not include "www" that is the sub-domain. It is a folder on your website. You can have multiple sub-domains all under the same domain like
"www.mywebsite.com" "blog.mywebsite.com" "forum.mywebsite.com" all on one domain name "mywebsite.com"
Some websites have no sub-domain because the "www" has been removed using a file named "htaccess"