A subdomain is a part of a web address that is under the main domain name, for instance name.example.com. From a technical perspective, even in www.example.com the "www" part is a subdomain since the fully qualified domain name is only "example.com". Each subdomain could have its own website and records and can even be hosted with a different provider if you wish to use a specific feature which isn't provided by your current service provider. An illustration for using a subdomain is if you have a business site along with an online store under a subdomain where customers can purchase your products. You can also have a forum in which they can discuss the products and by employing subdomains rather than subfolders you are going to 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 less risky in the case of a script security breach.