An addon domain allows a new domain name to link to a subdirectory of your account. The addon domains files will be stored in a subdirectory of your public_html folder. Creating an addon domain gives you the ability to hosting multiple domains from a single hosting account
How to create an addon domain ?
1. Navigate to cPanel => Add-on Domains ( under domain section in your cPanel)
2. Fill the Details about new domain and click on create.
Video - http://tutorials.lankahost.net/cpanel-tutorials/cpanel1132x3-24-addon.swf
Note - Before you create add-on domain, your 2nd domain must be pointed to same name servers as your primary domain name
