A subdomain is part of a web address which is under the main domain name, 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". Every subdomain could have its own site and records and can even be hosted through a different company if you want to use a certain feature that isn't provided by your current provider. One example for using a subdomain is if you have a company website along with an online store under a subdomain where customers can purchase your products. In addition, you can have a forum in which they can comment on the products and by employing subdomains as opposed to subfolders you are going to avoid any probability of all websites going down when you perform maintenance, or update one of the website scripts. Keeping your websites separated is less risky in case of a script security breach.