This allows you to have several separate Web sites on your account. You can have a mainsite.com and then a example.com as an add-on domain. The example.com would be a completely separate Web site and visitors would not know that it is part of your mainsite.com account.
A Parked Domain is a domain name that points to your main Web site on your account. For example, if garanntor.com is your main Web site, you can purchase the garanntor.net and assign it as a parked domain so that if a visitor were to type garanntor.net they would see the same Web site as if they had typed cars.com.
A subdomain is a second website with its own unique content, but there is no new domain name. Instead, you use an existing domain name and change the www to another name. The subdomain name looks like apps.domain.com, help.domain.com, help2.domain.com (assuming you already host domain.com).