A subdomain is part of a web address that is under the main domain, for instance name.example.com. Technically, even in www.example.com the "www" part is a subdomain because the fully qualified domain name is just "example.com". Each subdomain can have its own website and records and can also be hosted through a different provider if you want to use a feature that is not provided by your current service provider. An example for using a subdomain is if you have a company site as well as an online store under a subdomain where customers can buy your products. In addition, you can have a forum in which they can comment on the products and by using subdomains instead of subfolders you'll avoid any chance of all sites going down if you perform maintenance, or update one of the site scripts. Keeping your websites separated is also more secure in case of a script security breach.