A subdomain is a part of a web address that is under the main domain, for example name.example.com. Actually, 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 site and records and can even be hosted through a different company if you need to use a specific feature that isn't provided by your current provider. An illustration for using a subdomain is if you have a company website 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 discuss the products and by using subdomains rather than subfolders you'll avoid any probability of all websites going down when you perform maintenance, or update one of the site scripts. Keeping your websites separated is also less risky in the event of a script security breach.