An Add-on Domain is similar to a sub-domain and created in the same way. An Add-on Domain has its own file path, location, and file set that can be used to store a web site, images, documents or anything you want. It is almost like having your own web space for a different domain, but is attached to your main user account. This allows for easier management of the Add-on Domain by being integrating and available from your main user account’s management system.
If you manage multiple web sites and want to control them under one simple plan, you will need add-on domains support. With an add-on domains you may use either a sub-domain of your primary domain name (example: something.abc.com) or you can use an entirely different domain. Add-on domains have their own unique web site, emails, CGI, PHP, MySQL, Stats, etc. for each add-on domains, all connected by a single control panel.