In cPanel/WHM terms, an add-on domain is a VirtualHost with its own set of log files, stats, etc, and is treated as a subdomain. A parked domain is simply an alias to an already existing domain.
For example, if:
test.example.com, is an add-on domain for example.com - it will contain its own files and is a unique website.
mirror.example.com is a parked domain - it will not have its own unique files or logs, and the domain itself points to some other site, such as test.example.com or example.com or some.example.com, etc.
