A domain name is an identification label based on the Domain Name System (DNS). Labels are made up of letters, numbers and hyphens, and may not start with hyphens. Domain Names are organized in subordinate levels starting at the top the root domain. The next level is the top level domains or TLD's which include .com, .net, .org and country code TLD's. The next level down are the domain names which are handled by registrars that you and I can register for websites.
See also:
Label
TLD
Root