A subdomain is part of a web address that is under the main domain, such as name.example.com. From a technical perspective, even in www.example.com the "www" element is a subdomain given that the fully qualified domain name is only "example.com". Every single subdomain can have its own site and records and can even be hosted through a different provider if you wish to use some feature that's not offered by your current provider. One example for using a subdomain is if you have a business site as well as an online store under a subdomain where customers can buy your products. You can even have a forum in which they can discuss the products and by employing subdomains as an alternative to subfolders you'll avoid any chance of all websites going down if you perform maintenance, or update one of the site scripts. Keeping the websites separated is more secure in the event of a script security breach.