The Add-on domains feature lets you host several websites from a single hosting account, they will all be controlled through the same control panel.
The difference between Add-on domains and Parked domains is that Add-on domains don't have to be pointed at the root of the domain, they act as completely seperate domains, nobody will ever know there are mulitple ddomains hosted on the account.
Add-on domains must be registered domain names that you own and the nameservers must be set to match that of the main domain on your account.
To add-on a domain to your existing control panel:
1. Login to your Control Panel
2. Click on the Addon Domains button on the home page.
3. Enter the domain name in the New Addon Domain field, i.e. domain2.com (without http://www)
4. Enter a username for the new add-on domain, i.e. domain2 (a folder will also be created with this name for you to upload a site)
5. Enter the password for the domain in the Password field.
6. Click on the Add button.
You are then ready to upload the new website, upload it to /public_html/domain2/, once the DNS has propagated you will be able to view by the domain name, i.e. http://www.domain2.com
You will also be able to configure email and view statistics for your add-on domains.