What is a sub domain?

www.domain.com is a domain name.  A sub domain is anything that replaces the www in a domain name. forum.domain.com is a subdomain. www.forum.domain.com is not a sub domain

Sub Domain Setup 

To create a sub domain cPanel asks for the name you wish to use. MAKE SURE the sub domain name you give it doesn’t have a folder already named that name. If it does the sub will not work. Example: I want to create the sub domain test.hostgator.com so my sub name is going to be test. If I already have a folder in my ftp named test I need to delete it or rename it to something different before I create the sub domain with the test name from cPanel