A subdomain is a part of a web address that is under the main domain name, such as name.example.com. Actually, even in www.example.com the "www" part is a subdomain because the fully qualified domain name is only "example.com". Each subdomain can have its own website and records and can even be hosted with a different provider if you want to use a certain feature which is not provided by your current provider. A good example for using a subdomain is if you have a company website along with an online store under a subdomain where clients can acquire your products. You can even have a forum in which they can discuss the products and by employing subdomains instead of subfolders you will avoid any chance of all sites going down when you perform maintenance, or update one of the site scripts. Keeping the websites separated is also less risky in the event of a script security breach.