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 phones.com and then a cars.com as an add-on domain. The cars.com would be a completely separate Web site and visitors would not know that it is part of your phones.com account.
A Parked Domain on the other hand is a domain name that points to your main Web site on your account. For example, if cars.com is your main Web site, you can purchase the cars.net and assign it as a parked domain so that if a visitor were to type cars.net they would see the same Web site as if they had typed cars.com.