A subdomain is a part of a web address that's under the main domain, for instance name.example.com. From a technical perspective, even in www.example.com the "www" part is a subdomain because the fully qualified domain name is only "example.com". Every subdomain can have its own website and records and can even be hosted with a different provider if you need to use a certain feature that isn't provided by your current service provider. An example for using a subdomain is if you have a business site as well as an online store under a subdomain where customers can buy your products. You can even have a forum in which they can discuss the products and by employing subdomains as opposed to subfolders you are going to avoid any risk of all websites going down if you perform maintenance, or update one of the website scripts. Keeping your sites separated is also less risky in the event of a script security breach.