A subdomain is a part of a web address that is under the main domain, such as name.example.com. Technically, even in www.example.com the "www" element is a subdomain because the fully qualified domain name is just "example.com". Every subdomain can have its own site and records and can also be hosted with a different provider if you wish to use a specific feature that is not offered by your current provider. A good example for using a subdomain is if you have a company site along with an online store under a subdomain where customers can buy your products. You can also have a forum in which they can discuss the products and by using subdomains as an alternative to subfolders you will avoid any chance of all sites going down when you perform maintenance, or update one of the site scripts. Keeping the websites separated is also less risky in the event of a script security breach.