A subdomain is part of a web address which is under the main domain name, for example name.example.com. Actually, even in www.example.com the "www" part is a subdomain since the fully qualified domain name is just "example.com". Each subdomain can have its own web site and records and can also be hosted using a different company if you need to use a feature that isn't provided by your current service provider. An example for using a subdomain is if you have a business site as well as an online store under a subdomain where clients can acquire your products. In addition, you can have a forum in which they can comment on the products and by employing subdomains rather than subfolders you'll avoid any chance of all websites going down if you perform maintenance, or update one of the site scripts. Keeping the websites separated is also less risky in the event of a script security breach.