A subdomain is a part of a web address that's under the main domain name, for example name.example.com. Technically, even in www.example.com the "www" element is a subdomain because the fully qualified domain name is only "example.com". Each subdomain could have its own website and records and can also be hosted with a different company if you wish to use a specific feature which is not provided by your current service provider. An illustration 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 in which they can talk about the products and by employing subdomains instead of subfolders you are going to avoid any risk of all websites going down if you perform maintenance, or update one of the site scripts. Keeping your sites separated is also more secure in the event of a script security breach.