From Hostgator – see this link for 1:35 video
It is not always necessary to register a new domain name when the one you already own will work perfectly fine. Rather than registering a new domain name, you can always create a subdomain using a domain you already own.
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, (assuming you already host or own domain.com).