An addon domain is a fully functional domain that can be created from your cPanel control panel. Once it's created it will have it's own email, and it's own folder from within the public_html directory.
Addon domain url's look like www.domain.com
Sub domain url's look like forum.domain.com etc.domain.com the www is replaced with anything you want.
Parked domains are basically masked domains, domainB.com will load the same content as domainA.com if domainB.com is parked on top of DomainA.com