The following example illustrates the difference between a URL (Uniform Resource Locator) and a domain name:

URL: http://www.example.net/index.html
Domain name: www.example.net
Registered domain name: example.net

As a general rule, the IP address and the server name are interchangeable. For most Internet services, the server will not have any way to know which was used. However, the explosion of interest in the Web means that there are far more Web sites than servers. To accommodate this, the hypertext transfer protocol (HTTP) specifies that the client tells the server which name is being used. This way, one server with one IP address can provide different sites for different domain names. This feature goes under the name virtual hosting and is commonly used by web hosts.
 

Original content: http://en.wikipedia.org/wiki/Domain_name