What is a subdomain name
From Bravenet Wiki
A subdomain is a domain name that is based off of your main domain. An example of a subdomain name is support.bravenet.com. That subdomain name is a sub-domain of bravenet.com.
Subdomains allow you to create a different section of your website which users can access very quickly using the subdomain name, but also allows you to develop the two sections separately.