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 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. Please read before creating an Addon Domain.
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 you need a place to park a domain you do not have a website for, when you have more than one domain that should lead to your primary domain, or when you have common misspellings of your domain name that you have registered.
For example, if you own mydomain.com and it is the primary domain of your cPanel account, you can set up ourdoman.com as a parked domain and it will load the content of mydomain.com. You will notice that although the content of mydomain.com is loading in the browser, the address bar will display http://ourdomain.com/.