A subdomain is a part of a web address which is under the main domain name, for instance name.example.com. Actually, even in www.example.com the "www" part is a subdomain given that the fully qualified domain name is only "example.com". Every subdomain can have its own web site and records and can even be hosted through a different company if you need to use a certain feature that is not provided by your current service provider. An illustration for using a subdomain is if you have a company site and an online store under a subdomain where customers can buy your products. In addition, you can have a forum in which they can talk about the products and by using subdomains rather than subfolders you'll avoid any probability of all sites going down if you perform maintenance, or update one of the website scripts. Keeping the sites separated is also less risky in the event of a script security breach.