A subdomain is a part of a web address which is under the main domain, for instance name.example.com. Actually, even in www.example.com the "www" element is a subdomain since the fully qualified domain name is just "example.com". Every subdomain can have its own web site and records and can also be hosted using a different provider if you wish to use a specific feature which is not provided by your current provider. An example for using a subdomain is if you have a company website along with an online store under a subdomain where customers can buy your products. Additionally, you can have a forum in which they can discuss the products and by employing subdomains as opposed to subfolders you'll avoid any chance of all sites going down if you perform maintenance, or update one of the website scripts. Keeping your sites separated is less risky in the event of a script security breach.