Domain: The domain identifies the name of the site.

It comprises two parts:  the proper name of the site and, separated by a dot, an abbreviation indicating the name of the country or the category of the site.  In the case of the URL www.example.com, the name of the domain is “Example” while “.com” is the short form for “commercial”.  Beside the main (top level) domain, in this case, “com”, there may be others, known as sub-domains of second or third level (“second-level.example.com”, “third-level.example.com”), always separated from the first level by a dot.

A DNS (domain name server) associates each domain with an  IP address,  the code that allows the “net” to recognize a certain site.  If you want a site to be visible on the Internet, the domain must always be registered.