A subdomain is a part of a web address that is under the main domain, for instance name.example.com. Technically, even in www.example.com the "www" element is a subdomain because the fully qualified domain name is just "example.com". Every subdomain could have its own web site and records and can also be hosted using a different provider if you need to use a certain feature which isn't provided by your current service provider. An example for using a subdomain is if you have a company website along with an online store under a subdomain where clients can acquire your products. You can also have a forum where they can comment on the products and by employing subdomains rather than subfolders you'll avoid any risk of all sites going down if you perform maintenance, or update one of the site scripts. Keeping your sites separated is less risky in the case of a script security breach.