A subdomain is part of a web address that is under the main domain name, for instance name.example.com. Technically, even in www.example.com the "www" part is a subdomain given that the fully qualified domain name is only "example.com". Every single 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 is not 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 customers can acquire your products. You can also have a forum where they can discuss the products and by employing subdomains as opposed to subfolders you will 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 the case of a script security breach.