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 giggle.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.