A subdomain is part of a web address that is under the main domain name, for example name.example.com. From a technical perspective, even in www.example.com the "www" element is a subdomain because the fully qualified domain name is just "example.com". Every subdomain can have its own website and records and can also be hosted with a different provider if you want to use a feature which is not provided by your current provider. An illustration for using a subdomain is if you have a business site along with an online store under a subdomain where customers can buy your products. Additionally, you can have a forum in which they can talk about the products and by employing subdomains instead of subfolders you'll avoid any risk of all websites going down if you perform maintenance, or update one of the website scripts. Keeping the sites separated is also less risky in the event of a script security breach.