@LeeUJan 16.2009 — #Each is a regular, registered domain. It is then pointed to your main domain. Then, it is added as a subdomain on your site. The method used depends on your Web host.
@ahk2chanJan 20.2009 — #Once sub-domains are set up, my hosting company will actually automatically create a folder with the same name as the sub-domain right under root, so that sub.domain.com will be served from domain.com/sub/.