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