One method is called parking a domain and is simply an alternate domain name to access your current content you have hosted at your primary domain. Both domains would 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 "Parked Domains".
The other is called an "Addon Domain" and it points to a directory within your hosting. You can set what location you would like your addon domain to point to. Go to the Domains Section and click on "Addon Domains".