Addon domain : An addon domain is a fully functional domain that can be created from your cpanel. 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 : sub domain url's look like forum.domain.com sub.domain.com etc. the www is replaced with any name you want.
Parked domains : Parked domains are a place holder site. You park it on top of another site and it becomes a single page of the site you park it on.