A subdomain is a part of a web address which is under the main domain name, such as name.example.com. From a technical perspective, even in www.example.com the "www" part is a subdomain since 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 through a different company if you wish to use a certain feature which is not provided by your current provider. An example for using a subdomain is if you have a business site as well as an online store under a subdomain where customers can buy your products. You can even have a forum in which they can discuss the products and by employing subdomains rather than subfolders you'll avoid any chance of all sites going down when you perform maintenance, or update one of the website scripts. Keeping the sites separated is less risky in case there is a script security breach.