Your primary domain is the main domain name on the account. It’s the domain you use when you first order your hosting service and normally is the domain most people will use to access your site. The primary domain serves the files located in your public_html folder.
Addon domains allow you to add additional independent domains that can serve files from any directory on your account.
Parked domains in essence sit on top of your primary domain and serves the same contact your main/primary domain does (files in public_html).
Subdomains are extensions of the other types of domains. For example, if you had example.com as your primary domain, you could create sub.example.com as a subdomain.