Basically, website forwarders, aliases, and pointers are all ways to have additional domains directed to your same web site. For example, you might have somedomain.com as your website. If you wanted somedomain.net to also be directed to that same web site, you would use a website alias.
The related FAQ listed below on adding extra sites to ProWeb is important to read: Adding extra sites to ProWeb
It's also important to be familiar with the distinctions between registration, DNS, and hosting. We have a nice FAQ on the subject that you can read
Please be specific when you ask questions about adding extra sites and the related registration issues.