A subdomain is part of a web address that is under the main domain, 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 through a different company if you wish to use some feature that is not provided by your current service provider. An example for using a subdomain is if you have a company website and an online store under a subdomain where customers can buy your products. You can also have a forum where they can comment on the products and by employing subdomains as an alternative to subfolders you'll avoid any risk of all websites going down when 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.