A subdomain is a 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". Each subdomain can have its own web site and records and can also be hosted with a different company if you want to use some feature that isn't offered by your current provider. One example for using a subdomain is if you have a company website along with an online store under a subdomain where customers can acquire your products. Additionally, you can have a forum in which they can comment on the products and by employing subdomains as opposed to subfolders you'll avoid any probability of all sites going down when you perform maintenance, or update one of the site scripts. Keeping the sites separated is also more secure in the event of a script security breach.