A subdomain is part of a web address that is under the main domain name, for instance name.example.com. Actually, even in www.example.com the "www" element is a subdomain given that the fully qualified domain name is just "example.com". Each subdomain could have its own website and records and can even be hosted through a different company if you want to use a specific feature that is not provided by your current provider. One example for using a subdomain is if you have a business website and an online store under a subdomain where customers can buy your products. You can even have a forum in which they can talk about the products and by employing subdomains as an alternative to subfolders you'll avoid any risk of all sites going down if you perform maintenance, or update one of the site scripts. Keeping your sites separated is more secure in case there is a script security breach.