An Addon Domain is a domain name which points to its own folder within public_html and appears as a separate website from your primary domain.
This allows you to have two separate Web sites on your account. You can have a abc.com and then a xyz.com as an add-on domain. The xyz.com would be a completely separate Web site and visitors would not know that it is part of your abc.com account.
Parked Domains
A Parked Domain is a domain name that points to your main Web site on your account
For example, if abc.com is your main Web site, you can purchase the abc.net and assign it as a parked domain so that if a visitor were to type abc.net they would see the same Web site as if they had typed abc.com.