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 URLs look like www.domain.com Subdomain URLs look like help.domain.com, stuff.domain.com, etc. domain.com the www is replaced with anything you want. 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.