An Introduction to Sub-Domain Names

A subdomain or sub-domain is also known as a "third-level" domain, and is a way to create memorable web addresses for various areas of your site. For example http://mail.yoursite.com or http://help.yoursite.com.

Notice the three domain levels in the chart below:

Here's an example:
Our site is www.alienhosts.com, and we created a subdomain for help files. We set it up, the address would be help.alienhosts.com. This would go to a different area of the site in a folder called help.