A subdomain is a domain using your main domain name and a prefix name. As an example, let's say our main domain is cpx3demo.com. We want to create a domain for our support department and direct visitors to a separate area on our web site. Our subdomain would use "support" as the prefix and "cpx3demo.com" as the main domain. The example of putting these two together are shown below:
Consider a subdomain a url for any area on your web site. You can create the prefix as any name you want based on your own personal preferences.