A subdomain is a part of a web address that's under the main domain name, such as name.example.com. Actually, even in www.example.com the "www" element is a subdomain since the fully qualified domain name is just "example.com". Every subdomain can have its own web site and records and can even be hosted through a different company if you wish to use a feature which is not offered by your current 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 buy your products. In addition, you can have a forum where they can talk about the products and by using subdomains as opposed to subfolders you will avoid any risk of all sites going down when you perform maintenance, or update one of the site scripts. Keeping the websites separated is also more secure in the event of a script security breach.