Subdomain: A subdomain is a site with totally different content but with no new domain name. It's a directory inside public_html folder in your server. For example: mydomain.com/mydirectory or mydirectory.mydomain.com. This facility is free of cost. You simply needs to create the directory inside public_html folder in which you upload your site files using FTP.

Parked Domain : Parked domain is the unique domain name that points the primary domain. The content of the parked domain is not unique. It simply points the main site. You need to register the domain name in this case.

Addon Domain: Addon Domain is completely different sites. It has its own unique contents. You need to purchase a new domain name for this. Then you can add this domain name by your cpanel.