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 domains load their content from a subdomain folder of the primary domain name. For instance, if you own domain.com and wish to addon example.com, once you have successfully added example.com, you may load it in one of three ways.
The addon domain, example.com, can be loaded in the following ways:
example.com - This is loading the domain as an addon domain.
example.domain.com - This is loading the addon as a subdomain.
domain.com/example - This is loading the addon as a subfolder of the primary domain.
A parked domain is a masked forward to the primary domain name of the cPanel account. It is much like a place holder site. For instance, if you owned google.com and it was the primary domain of your cPanel account, you could set up gewgol.com as a parked domain and it would load the content of google.com. You will notice that all though the content of google.com is loading in the browser, the address bar will display http://gewgol.com/.
Subdomain URLs look like forum.domain.com, cat.domain.com, etc.domain.com the www is replaced with anything you want.