A subdomain is part of a web address that's under the main domain name, such as name.example.com. Technically, even in www.example.com the "www" part is a subdomain because the fully qualified domain name is just "example.com". Each subdomain can have its own website and records and can also be hosted using a different company if you need to use a certain feature that isn't offered by your current provider. An example for using a subdomain is if you have a business website along with an online store under a subdomain where clients can purchase your products. In addition, you can have a forum where they can comment on the products and by employing subdomains instead of subfolders you will avoid any risk of all sites going down if you perform maintenance, or update one of the site scripts. Keeping your sites separated is also less risky in case there is a script security breach.