A subdomain is a part of a web address that is under the main domain, such as name.example.com. Technically, 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 website and records and can even be hosted through a different company if you wish to use a certain feature that is not provided by your current provider. An example for using a subdomain is if you have a business website as well as an online store under a subdomain where customers can buy your products. In addition, you can have a forum where they can comment on the products and by employing subdomains instead of subfolders you'll avoid any risk of all sites going down if you perform maintenance, or update one of the website scripts. Keeping the websites separated is less risky in case of a script security breach.