What is a Sub domain? In order to understand what a sub domain is, first let's look at a regular domain. Most domains look like the following; www.wordpress.com. Each domain refers to a specific site on the web and the search engines acknowledge each domain as a separate web site. While many domains have several web pages, each extra page is still considered the same site (for instance, www.wordpress.com/UserName). While this is an easy way to add extra pages to your web site, for most web deve...