A subdomain is part of a web address that is under the main domain, for example 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". Each subdomain could have its own website and records and can even be hosted through a different provider if you need to use a feature that is not offered by your current service provider. An example for using a subdomain is if you have a company site as well as an online store under a subdomain where customers can buy your products. Additionally, you can 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 your sites separated is more secure in the case of a script security breach.