A subdomain is part of a web address which is under the main domain, for instance name.example.com. From a technical perspective, even in www.example.com the "www" part is a subdomain since the fully qualified domain name is just "example.com". Every single subdomain can have its own website and records and can also be hosted with a different provider if you wish to use a specific feature that is not offered by your current service provider. A good 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. You can also have a forum where they can comment on the products and by employing subdomains as opposed to subfolders you are going to avoid any probability of all sites going down if you perform maintenance, or update one of the site scripts. Keeping the sites separated is also more secure in case of a script security breach.