A subdomain is a part of a web address which is under the main domain, such as name.example.com. Technically, even in www.example.com the "www" element is a subdomain because the fully qualified domain name is only "example.com". Every subdomain could have its own website and records and can also be hosted through a different company if you wish to use a certain feature that is not provided by your current service provider. An example for using a subdomain is if you have a company website as well as an online store under a subdomain where customers can purchase your products. You can also have a forum where they can discuss the products and by employing subdomains instead of subfolders you will avoid any chance of all sites going down if you perform maintenance, or update one of the website scripts. Keeping the sites separated is less risky in case there is a script security breach.