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 forums.domain.com, help.domain.com, help2.domain.com (assuming you already host domain.com).
An addon domain is a second website, with its own unique content. This type of domain does require you to register the new domain name before you can host it. So, if you want to host www.otherdomain.com, this is the solution. For additional information regarding addon domains, please refer to the following article:
A parked domain is NOT a unique website. Instead, it is a masked forward to the primary domain name of your cPanel account, much like a website alias. Parked domains are commonly used when: