A subdomain is part of a web address which is under the main domain, for example name.example.com. Actually, even in www.example.com the "www" element is a subdomain given that the fully qualified domain name is only "example.com". Each subdomain can have its own web site and records and can also be hosted through a different provider if you need to use a specific feature that is not provided by your current provider. An illustration 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. You can also have a forum where they can discuss the products and by using subdomains as opposed to subfolders you are going to avoid any risk of all sites going down when you perform maintenance, or update one of the site scripts. Keeping the sites separated is also more secure in case there is a script security breach.