A subdomain is a part of a web address that's under the main domain, for example name.example.com. Actually, even in www.example.com the "www" element 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 using a different provider if you wish to use a feature that's not provided by your current provider. A good example for using a subdomain is if you have a business site and an online store under a subdomain where customers can buy your products. You can also have a forum where they can discuss the products and by employing subdomains as opposed to subfolders you are going to avoid any chance 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.