What is the Difference between ParkedDomain and AddonDomain and SubDomain ?

Addon Domain:
                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.Every addon domain is also a subdomain. 

Addon domains are handled similarly to subdomains in terms of the file structure listed in your file directory; however, the addon domain URL appears like a primary domain. In general, nothing that happens in your addon domain's files or DNS should affect what goes on with the rest of your domains.

You may actually load your addon domain using any of the following URLs:

Addon Domain Name
http://addondomain.com/

Subdomain Name
http://addondomain.example.com/

Subfolder Name
http://example.com/addondomain.com/

Sub Domain:
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 blog.example.com, help.example.com, support.example.com (if you have already hosted example.com).


Parked 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 for which you do not have a website
You have more than one domain that should lead to your primary domain
You have common misspellings of your domain name that you have registered