The main difference bewteen the addon domain and the parked domain is that the addon domain allows you to host more than one website on your account, while the parked domain is pointed to the main website on the account.
With the help of the addon domain, you will be able to host a second website in the same account that will share the resources of that account.
The parked domain is a domain pointed to the main website in your account. It is not associated with an independent website, but opens the same content as the primary domain for your account. To learn more about the parked domains, please click here.