A subdomain is a part of a web address which is under the main domain name, for example 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". Every single subdomain can have its own web site and records and can even be hosted through a different company if you need to use some feature that's not provided by your current service provider. An example for using a subdomain is if you have a company website as well as an online store under a subdomain where customers can acquire your products. Additionally, you can have a forum in which they can discuss the products and by using subdomains rather than subfolders you will avoid any chance of all websites going down when you perform maintenance, or update one of the site scripts. Keeping the sites separated is more secure in case there is a script security breach.