No it is not. It's just a handy way to indicate that the URL is a Web server rather a FTP server. Some websites contain server side scripts to add or remove the 'www' prefix from their address. Microsoft, for example, add the 'www' prefix to their address if you type their URL address without it.
Tim Berners-Lee has indicated that the widespread use of the 'www' subdomain prefix was unintended. Apparently Berners-Lee was going to use the 'info' prefix instead of the 'www' prefix for the CERN website, but never updated the DNS information. As CERN and Berners-Lee were at the heart of the World Wide Web, subsequent websites copied using the 'www' prefix from the CERN website.
However, present day, in 2012, you can see that the CERN website now uses a server side redirect to switch users from the 'www' subdomain prefix to a 'public.web' subdomain prefix. Quite a few websites prefer to remove the 'www' prefix because it makes it simpler for the user to type their address, and makes their domain name more memorable and ideal for advertisement, if it is 'example.com' rather than 'www.example.com'.
While websites can use a server side redirect script to add or remove the 'www' prefix, it has been noted that some web browsers do attempt to always add the 'www' prefix to a website, even if it is not desired or requested by the user or website management.
Additional: WWW is an acronym that is short for the World Wide Web. The 'www' acronym is often used as a prefix/extension for the hostname of a web server. For example, in the following hostname: www.internet-guide.co.uk. What should be noted, is that, while 'www' is widely used as a hostname for web servers, there is no technical reason or requirement to do so. For example, Wikipedia does not use 'www' for the hostname of it's web server, instead it uses 'en': en.wikipedia.org. The only reason that the 'www' prefix is used in the vast majority of hostnames for web servers, is that, for advertising purposes etc, it clearly indicates that the URL (address) of the resource is a website.