There are a couple of ways that you can host additional domains - the first is called parking a domain and is simply an alternate domain name to access your current content you have hosted at your primary domain.
http://www.yourdomain.com/ and http://www.yourparkeddomain.com/ would both return the same results.
To park a domain log in to your control panel (http://www.yourdomain.com/cpanel/) and go to the Domains Section and click on Aliases.
The second method is called an "addon domain" and it points to a location within your hosting account, just as your primary domain points to a location within our server itself. As of cPanel version 11 you can set what location you would like your addon domain to point to. If you were to use /home/public_html/addondomain.com as the folder and your domain (addondomain.com for example) will show what is in your /home/public_html/addondomain.com folder.
To host an addon domain log in to your control panel (http://www.yourdomain.com/cpanel/) and go to the Domains Section and click on Addon Domains.