A subdomain is part of a web address that is under the main domain name, such as name.example.com. Technically, even in www.example.com the "www" part is a subdomain since the fully qualified domain name is just "example.com". Each subdomain can have its own web site and records and can even be hosted with a different company if you need to use a certain feature that isn't offered by your current service provider. A good example for using a subdomain is if you have a business site and an online store under a subdomain where clients can acquire your products. You can also have a forum in which they can discuss the products and by employing subdomains instead of subfolders you'll avoid any chance of all sites going down if you perform maintenance, or update one of the website scripts. Keeping the sites separated is more secure in case of a script security breach.