A sub domain is an extension of your main domain. For example, if you have a domain that is named "mydomain.com" and you wanted to have a URL of "mysub.mydomain.com", the "mysub" part is the sub domain. You can use a sub domain for many things but most people use the sub domain to create a section of your web site dedicated to one particular subject. If you had a web site dealing with cars, you could create a sub domain called "ford" to be used as a section of your web site that only dealt with cars and trucks that were made by Ford. The ability to create sub domains can really help your web site stay organized for easy searching. When a sub domain is created it is similar to having a second site set up. Usually you can provide FTP access to the sub domain. This can actually allow you to set up a test portion of your web site (e.g., test.mydomain.com) so that you can test your site without impacting your main site.