Obviously, on the Internet, there's a lot in a name. Such a name actually identifies a computer network associated with an organization. For example, unt.edu is part of all the addresses used within the University of North Texas' Internet network. In Internet terminology, such a collection of addresses is called a domain. Domains are organized in hierarchies.
Domain names can have as many hierarchical levels as necessary.
For example, .acs.unt.edu is the Academic Computing Services domain at UNT, .cas.unt.edu is the College of Arts and Sciences domain, etc. Each subdomain can have further levels of hierarchy as necessary to specify the specific services offered by that organization via the Internet network. Domain names are only half the story when you are talking about internet addresses. Names are convenient for people, but are not as useful to computer systems. Computer systems are just big calculators at heart and work much more efficiently with numbers than with names.