A subdomain is part of a web address which is under the main domain name, such as name.example.com. From a technical perspective, even in www.example.com the "www" element 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 using a different provider if you need to use a certain feature which isn't provided by your current provider. A good example for using a subdomain is if you have a business site as well as an online store under a subdomain where customers can purchase your products. You can even have a forum in which they can discuss the products and by using subdomains as opposed to subfolders you'll avoid any risk of all sites going down if 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.