A subdomain is part of a web address that's under the main domain name, such as name.example.com. From a technical perspective, even in www.example.com the "www" element is a subdomain given that the fully qualified domain name is only "example.com". Every single subdomain can have its own website and records and can also be hosted using a different company if you need to use a feature that is not provided by your current service 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. In addition, you can have a forum in which they can talk about the products and by using subdomains as opposed to subfolders you'll avoid any chance of all sites going down when you perform maintenance, or update one of the site scripts. Keeping the websites separated is also less risky in the case of a script security breach.