The address or URL of a particular Web site, it is the text name corresponding to the numeric IP address of a computer on the Internet. For example: www.netlingo.com is the domain name for the numeric IP address "126.96.36.199." There is an organization called InterNIC that registers domain names for a fee, to keep people from registering the same name. To register a domain name, you can contact a company (such as Network Solutions, Inc.) or you can ask your ISP or hosting company to register names for you.
BTW: Did you know the most popular domain-length is 11 characters, and there are more than 500 63-character domains registered, including DIDYOUKNOWTHATYOUCANONLYHAVESIXTY - THREECHARACTERSINADOMAIN - NAME.com ;-) Internet trivia!