A subdomain is a part of a web address that's under the main domain name, for example 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 also be hosted using a different provider if you want to use a certain feature that isn't offered by your current provider. An illustration for using a subdomain is if you have a business site as well as an online store under a subdomain where customers can buy your products. You can also have a forum where they can comment on the products and by using subdomains rather than subfolders you'll avoid any chance of all sites going down when you perform maintenance, or update one of the website scripts. Keeping the sites separated is also less risky in case of a script security breach.