A subdomain is a part of a web address which is under the main domain, such as name.example.com. Actually, even in www.example.com the "www" element is a subdomain since the fully qualified domain name is only "example.com". Each subdomain can have its own web site and records and can also be hosted through a different provider if you want to use a specific feature which isn't provided by your current service provider. An example for using a subdomain is if you have a company website as well as an online store under a subdomain where customers can purchase your products. You can even have a forum where they can discuss the products and by employing subdomains as opposed to subfolders you are going to avoid any risk of all sites going down if you perform maintenance, or update one of the website scripts. Keeping the sites separated is also more secure in the case of a script security breach.