A subdomain is part of a web address which is under the main domain, for instance name.example.com. Technically, even in www.example.com the "www" part is a subdomain given that the fully qualified domain name is just "example.com". Every single subdomain could have its own web site and records and can also be hosted through a different company if you need to use a feature that isn't offered by your current service provider. An example for using a subdomain is if you have a company site along with an online store under a subdomain where customers can acquire your products. In addition, you can have a forum in which they can discuss the products and by employing subdomains as opposed to subfolders you will avoid any probability of all sites going down if 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.