The quick answer to this question is that there is no difference between the two addresses for most modern domains. For example, typing www.wisegeek.com or wisegeek.com into the Uniform Resource Locator (URL) of your Web browser will bring you to this site with equal ease. However, leaving “www” off of some websites can result in the browser being unable to find the site. This problem is correctable by the domain holder. A cursory understanding of how the World Wide Web (WWW) works will be helpful in understanding the problem.
The Internet is a massive network of computers that communicate by using agreed upon protocols. For example, every computer on the Internet is assigned a unique numerical address so that information can be sent and received without being lost. These unique addresses are called Internet Protocol addresses, or IPs for short. In the case of a website the numeric IP maps to a name, as names are easier for surfers to remember than numbers.
The Domain Name System (DNS) database contains a record for each website, which stores the website's name and IP address. When clicking on a link or entering an address in a Web browser, it sends a request to the DNS database to resolve the name to the corresponding IP address. If the “www” prefix is left off and the browser stalls, it is likely that the DNS record does not contain the short version of the domain name: the version without the “www.”
Once the name is found in the DNS database it is resolved to the corresponding IP. This allows the browser to create a connection with the server that hosts the site. It requests the page and supplies your IP address, akin to sending a self-addressed stamped envelope. The host server sends the webpage to your computer and the transaction is complete.
In the past many host servers created websites as subdomains under www., following then-current naming conventions. "WWW" identified the server as a Web server, versus a server dedicated to other tasks. As the Internet became more widely traveled by the general public, however, the ubiquitous “www” was often overlooked when entering website addresses into browsers. This resulted in lost website traffic and frustrated surfers, as many DNS records only contained www.example.com, and not example.com for the domain name.
Over time hosts began dropping the “www” designation for Web servers, and domains were created as example.com. To catch traffic that might arbitrarily include “www” in the browser request, DNS records instead included an extra entry to cover this occurrence. CNAME is the DNS tag that maps an alias to the main name in the DNS record: in this case the "www." version of the name. With this solution surfers could include or exclude www. and reach the site either way.
DNS records can be modified to include a mapped alias. If you require this service for your domain(s), contact your domain registrar.