Source : Free On-Line Dictionary of Computing

top-level domain
     
         The last and most significant component of an
        {Internet} {fully qualified domain name}, the part after the
        last ".".  For example, {host} wombat.doc.ic.ac.uk is in
        top-level domain "uk" (for United Kingdom).
     
        Every other country has its own top-level domain, including
        ".us" for the U.S.A.  Within the .us domain, there are
        subdomains for the fifty states, each generally with a name
        identical to the state's postal abbreviation.  These are
        rarely used however.  Within the .uk domain, there is a .ac.uk
        subdomain for academic sites and a .co.uk domain for
        commercial ones.  Other top-level domains may be divided up in
        similar ways.
     
        In the US and some other countries, the following top-level
        domains are used much more widely than the country code:
     
        	.com - commercial bodies
        	.edu - educational institutions
        	.gov - U. S. government
        	.mil - U. S. armed services
        	.net - network operators
        	.org - other organisations
     
        Since the rapid commercialisation of the Internet in the 1990s
        the ".com" domain has become particularly heavily populated
        with every company trying to register its company name as a
        subdomain of .com, e.g. "netscape.com" so as to make it easy
        for customers to guess or remember the {URL} of the comany's
        {home page}.
     
        United Nations entities use the domain names of the countries
        where they are located.  The UN headquarters facility in New
        York City, for example, is un.org.
     
        Several new top-level domains are about to be added (Oct
        1997):
        	.nom   - individual people
        	.rec   - recreational organisations
        	.firm  - businesses such as law, accounting, engineering
        	.store - commercial retail companies
        	.ent   - entertainment facilities and organisations
     
        (1997-10-08)