A subdomain is part of a web address that's under the main domain name, such as 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". Each subdomain can have its own website and records and can even be hosted through a different provider if you need to use a feature that isn't provided by your current provider. An example for using a subdomain is if you have a company site along with an online store under a subdomain where clients can buy your products. You can even have a forum where they can discuss the products and by employing subdomains as opposed to subfolders you'll avoid any chance of all websites going down when you perform maintenance, or update one of the site scripts. Keeping the websites separated is more secure in case of a script security breach.