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" part is a subdomain because the fully qualified domain name is just "example.com". Every subdomain can have its own web site and records and can also be hosted through a different provider if you wish to use a certain feature that isn't provided by your current service provider. A good example for using a subdomain is if you have a company site as well as an online store under a subdomain where customers can purchase your products. You can also have a forum where they can talk about the products and by using subdomains rather than subfolders you will avoid any risk of all sites going down when you perform maintenance, or update one of the site scripts. Keeping your websites separated is more secure in case of a script security breach.