A subdomain is part of a web address which is under the main domain, for example name.example.com. From a technical perspective, even in www.example.com the "www" part is a subdomain since the fully qualified domain name is just "example.com". Every subdomain can have its own website and records and can even be hosted with a different company if you need to use some feature which is not provided by your current provider. A good example for using a subdomain is if you have a company website and an online store under a subdomain where customers can acquire your products. In addition, you can have a forum in which they can talk about the products and by employing subdomains instead of subfolders you will avoid any probability of all sites going down if you perform maintenance, or update one of the site scripts. Keeping your sites separated is less risky in the event of a script security breach.