A subdomain is part of a web address that's under the main domain, 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 just "example.com". Each subdomain could have its own web site and records and can even be hosted through a different company if you want to use a certain feature that isn't offered by your current service provider. One example for using a subdomain is if you have a business website as well as an online store under a subdomain where customers can buy your products. You can also have a forum in which they can discuss the products and by using subdomains as opposed to subfolders you'll avoid any risk of all sites going down if you perform maintenance, or update one of the website scripts. Keeping your sites separated is also less risky in the case of a script security breach.