A subdomain is part of a web address that is under the main domain name, for example name.example.com. Technically, even in www.example.com the "www" part is a subdomain because the fully qualified domain name is only "example.com". Every subdomain can have its own web site and records and can even be hosted with a different company if you wish to use some feature that isn't provided by your current service provider. A good example for using a subdomain is if you have a business website and an online store under a subdomain where customers can purchase your products. In addition, you can have a forum in which they can discuss the products and by using subdomains rather than subfolders you'll avoid any chance of all sites going down if you perform maintenance, or update one of the site scripts. Keeping the sites separated is also more secure in case there is a script security breach.