A subdomain is part of a web address that's 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 since the fully qualified domain name is just "example.com". Each subdomain could have its own web site and records and can also be hosted with a different provider if you need to use a specific feature that's not provided by your current provider. An example for using a subdomain is if you have a company website as well as an online store under a subdomain where clients can purchase 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 probability of all sites going down if you perform maintenance, or update one of the site scripts. Keeping the sites separated is less risky in case of a script security breach.