A subdomain is a second website with its own unique content, but there is no new domain name. Instead, you use an existing domain name and change the www to another name. The subdomain name looks like forums.domain.com, help.domain.com, help2.domain.com (assuming you already host domain.com).
When you create a sub domain for your web hosting, a sub folder is created in the main directory of your web site. Upload your files intended for this sub domain into this folder for them to appear correctly.