A subdomain is a part of a web address that 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 because the fully qualified domain name is only "example.com". Every single subdomain could have its own site and records and can even be hosted using a different provider if you wish to use some feature that isn't offered by your current provider. An example for using a subdomain is if you have a business site along with an online store under a subdomain where clients can buy your products. In addition, you can have a forum in which they can discuss the products and by using subdomains as an alternative to subfolders you'll avoid any probability of all websites going down if you perform maintenance, or update one of the site scripts. Keeping the sites separated is less risky in the case of a script security breach.