A Parked Domain is another domain that "points" to your website.
A Parked Domain is a domain that is "masked and forwarded" to the primary domain name of your cPanel account (much like a website alias). Parked Domains are commonly used when you have more than one domain name and you want users to be directed to your primary domain, or when you have registered common misspellings of your domain name that you want to "point" to your primary domain.
For example, if you own yourdomain.com and it is the primary domain of your cPanel account, you can set up yourdomain.co.uk as a Parked Domain and it will load the content of yourdomain.com. You will notice that although the content of yourdomain.com is loading in the browser, the address bar will display http://yourdomain.co.uk. Please read our article about How to create a Parked Domain.
A Subdomain is another "virtual" website with its own unique content.
A Subdomain uses your existing domain - there is no new domain name. You use your existing domain name but add another "prefix" to it to make the Subdomain name. The Subdomain name will look like forums.yourdomain.com, help.yourdomain.com, blog.yourdomain.com, etc.
An Addon Domain is a different website, with its own unique content.
An Addon Domain requires you to register the new domain name before you can host it. So, if you want to host www.myotherdomain.com, this is the solution. Please read our article about How to create an Addon Domain before you attempt to do this.