Sub domain
Suppose your domain is domain.com
You install an E-Commerce and want to add a directory as domain.com/commerce/.
You can add the directory as a subdomain by adding a subdomain to your Control Panel.
Now you can access your E-Commerce as domain.com/commerce or commerce.domain.com
Parked Domain
You have several domains such as for example domain.com, domain.org, domain.biz and domain.net
domain.com is the domain of your website and you want to add domain.org, domain.biz and domain.net.
You want all these domains to be in the same place. When someone types in one of your domain names domain.com, domain.org, domain.biz or domain.net they will be connected to your same website. To have a Parked Domain, you need to purchase additional domains.
Add-On Domains (Linux Hosting Plan only)
You have two domains, domain.com and addondomain.com.
You want the two domains to be separate but still on the same hosting.
The way to add an Add-On Domain is to purchase an additional domain.