A subdomain is part of a web address that is under the main domain, for instance name.example.com. From a technical perspective, even in www.example.com the "www" element is a subdomain because the fully qualified domain name is only "example.com". Each subdomain could have its own site and records and can also be hosted with a different provider if you need to use a specific feature that is not offered by your current provider. An illustration for using a subdomain is if you have a business 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 talk about the products and by employing subdomains as an alternative to subfolders you'll avoid any probability of all sites going down if you perform maintenance, or update one of the site scripts. Keeping the sites separated is also less risky in the event of a script security breach.