A subdomain is part of a web address that is under the main domain name, for instance 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 single subdomain can have its own web site and records and can also be hosted through a different company if you want to use a specific feature that isn't offered by your current service 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 acquire your products. You can even have a forum in which they can talk about the products and by employing subdomains as opposed to subfolders you are going to avoid any probability of all websites going down if you perform maintenance, or update one of the site scripts. Keeping your sites separated is also less risky in the event of a script security breach.