An addon domain is a fully functional domain that can be created from your cPanel. Once it's created it will have its own email and its own folder from within the `public_html` directory. The addon domain URL looks like a regular URL. An example is www.domain.com.
Subdomain URLs look like forum.domain.com, cat.domain.com etc. Note: For domain.com, the www is replaced with anything you want.
Parked domains are a placeholder site. You park it on top of another site and it becomes a single page of the site you park it on.