A subdomain is a part of a web address that is under the main domain, for example name.example.com. Technically, even in www.example.com the "www" element is a subdomain given that the fully qualified domain name is only "example.com". Every subdomain could have its own web site and records and can even be hosted through a different provider if you wish to use some feature that's not provided by your current provider. One example for using a subdomain is if you have a business site and an online store under a subdomain where clients can buy your products. You can even have a forum in which they can discuss the products and by using 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 your sites separated is less risky in case there is a script security breach.