A subdomain is a part of a web address that's under the main domain name, for example name.example.com. Technically, even in www.example.com the "www" element 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 provider if you need to use a feature that's not provided by your current provider. An example for using a subdomain is if you have a business site along with an online store under a subdomain where customers can purchase your products. In addition, you can have a forum in which they can discuss the products and by employing subdomains instead of subfolders you will avoid any probability of all sites going down when you perform maintenance, or update one of the website scripts. Keeping your sites separated is also less risky in case of a script security breach.