An addon domain is a fully functional new registered 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 support.techlogicsolutions.com, sub.domain.com, etc.
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.