This is not a programming question, but something I was curious for a long time. I am a web developer myself, but do not know the answer to this question. I notice some websites have just http:// in their address, with no www. (eg. http-colon-//delicious) and other websites have www. in their address (eg. http-colon-//www-dot-yahoo-dot-com)
www part of the URL is a sub domain. It is completely at the discretion of the owner of the domain whether or not
www or any other sub domain is used.
There are many sites out there that prefer not to use
www and even some that think it should be deprecated. Check out http://no-www.org, who have "validated" thousands of sites that don't use the
www part of the domain.