A subdomain is part of a web address which is under the main domain name, for instance name.example.com. Technically, even in www.example.com the "www" element is a subdomain since the fully qualified domain name is only "example.com". Every single subdomain can have its own web site and records and can even be hosted with a different provider if you need to use a specific feature which isn't offered by your current service provider. An example for using a subdomain is if you have a business website and an online store under a subdomain where customers can buy your products. In addition, you can have a forum in which they can talk about the products and by employing subdomains as opposed to subfolders you will avoid any probability of all sites going down when you perform maintenance, or update one of the website scripts. Keeping the websites separated is also more secure in case there is a script security breach.