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 because the fully qualified domain name is only "example.com". Each subdomain could have its own web site and records and can also be hosted through a different provider if you wish to use some feature which is not provided by your current provider. A good example for using a subdomain is if you have a business website as well as an online store under a subdomain where customers can acquire your products. Additionally, you can have a forum where they can talk about the products and by employing subdomains instead of subfolders you will avoid any chance of all sites going down when you perform maintenance, or update one of the site scripts. Keeping the sites separated is also less risky in the event of a script security breach.