A subdomain is part of a web address which is under the main domain, for example name.example.com. From a technical perspective, 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 site and records and can also be hosted through a different provider if you want to use a certain feature that is not provided by your current service 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 purchase your products. Additionally, you can have a forum in which they can comment on the products and by employing subdomains instead of subfolders you are going to avoid any risk of all sites going down when you perform maintenance, or update one of the site scripts. Keeping the sites separated is less risky in case there is a script security breach.