A subdomain is part of a web address that's under the main domain, for example name.example.com. Technically, even in www.example.com the "www" part is a subdomain because the fully qualified domain name is just "example.com". Every single subdomain could have its own web site and records and can even be hosted with a different provider if you want to use a certain feature that isn't provided by your current service 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 purchase your products. Additionally, you can have a forum in which they can discuss the products and by employing subdomains instead of subfolders you will avoid any risk of all websites going down if you perform maintenance, or update one of the site scripts. Keeping the sites separated is also more secure in the case of a script security breach.