Why do some Web addresses begin with “http://www†while others omit the “www†altogether?
A Web address is made up of different parts. Moving backwards from right to left in the address www.nytimes.com, the “.com†is the top-level domain name, and refers to the type of site. (The popular “.com†name was originally intended for commercial sites, while “.edu†is for educational institutions, and so on.)
The middle part of the address, “nytimes†is the unique domain name for the site. Domain names need to be set up with an official registrar so they work with the Internet's Domain Name System; the HowStuffWorks site explains how all that works here.
The “www†is a subdomain, a specific area within the domain, but not all sites will use it. If the address works with or without the “www†prefix in the address, the site's administrator has mapped things out so that both addresses point to the location of the server's Web content. If the site has not been configured to automatically serve up the pages if you leave off the “www,†you usually get a message saying the site cannot be found. In that case, you need to type in the “www.â€
