Domain Registration:

On the Web, the domain name is that part of the Uniform Resource Locator (URL) that tells a domain name server using the domain name system (DNS) whether and where to forward a request for a Web page. The domain name is mapped to an IP address (which represents a physical point on the Internet).

Your Domain Name must be "leased" from a Registrar for a period of time usually on a per year basis. Current prices are approximately $30 per year.