Here’s an example: if you own the domain name domainnamecheck.net, then you can set up an e-mail address (e.g. email@example.com) or a www-site with the address www.domainnamecheck.net. As another example: www.ibm.com is IBM’s international Internet address of IBM’s presence in the World Wide Web, whereas www.ibm.de is the Internet address of IBM’s German branch. For an owner of a domain name it is certainly possible to grant other randomly chosen names “under” one’s domain name (third level domain, fourth level domain …): e.g. e-mail addresses like firstname.lastname@example.org.