A subdomain is part of a web address which is under the main domain name, for instance name.example.com. Technically, even in www.example.com the "www" part is a subdomain since the fully qualified domain name is only "example.com". Every subdomain can have its own site and records and can even be hosted through a different provider if you want to use some feature which is not provided by your current service provider. One example for using a subdomain is if you have a company website 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 discuss the products and by employing subdomains rather than subfolders you'll avoid any probability of all sites going down when you perform maintenance, or update one of the site scripts. Keeping the sites separated is less risky in the event of a script security breach.