What is the difference between Add-on domain/Subdomain/Parked domain?

You can create Add-on domains, subdomains and even Parked domains using cPanel.

1. Parked Domain:
Parked domain is a domain which is just "Parked" on to the main domain. Suppose that you have a domain maindomain.com with cPanel. Now you want xyz.com to serve as an alias for your "maindomain.com", so that whenever any one opens the xyz.com he'll be directed to the maindomain.com. Then first you need to update the nameservers for the domain : xyz.com so that they point to the same nameservers as the ones for maindomain.com.
Now go to the maindomain.com's cPanel. Click on the parked domains -> And add the domain xyz.com there. Now the domain xyz.com will be parked on the maindomain.com. Remember that you need to register/own the domain xyz.com before you do this.

2. Sub Domain:
A subdomain will be in the form sudomain.maindomain.com. When you create a subdomain using cPanel it will create a directory in your public_html folder.
Let's say your main domain is "maindomain.com" and the subdomain is "subdomain". Go to cPanel of "maindomain.com" and then click on Sub Domains give the name "subdomain" and then click on "Add" you can even setup a redirection for this domain from here. (If the directory name is "sub") The Subdomain can be accessed using: sub.maindomain.com or maindomain.com/sub/


3. Add-on Domain:
Add-on domain is almost same as Sub Domain but in case of Add-on domain the domain should be already registered by you and the nameservers for that add-on domain should be pointing the same nameservers as the maindomain.com. The add-on domain will also be hosted on a folder in the public_html folder. (in this case the folder name is addon) You can access the Add-on domain using: addon.com or maindomain.com/addon/ or addon.maindomain.com

An add-on domain is viewable by your visitors as a normal domain but you have the ability of managing multiple domains within one control panel.