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 because the fully qualified domain name is only "example.com". Each subdomain can have its own website and records and can also be hosted using a different provider if you need to use a feature that isn't provided by your current provider. One example for using a subdomain is if you have a business website and an online store under a subdomain where customers can acquire your products. You can also have a forum where they can comment on the products and by using subdomains as an alternative to subfolders you'll avoid any probability of all sites going down when you perform maintenance, or update one of the site scripts. Keeping the websites separated is more secure in case there is a script security breach.