A subdomain is a part of a web address that's under the main domain, for instance name.example.com. Technically, even in www.example.com the "www" element is a subdomain because the fully qualified domain name is just "example.com". Every subdomain could have its own website and records and can even be hosted using a different company if you want to use a feature which is not provided by your current service provider. An illustration for using a subdomain is if you have a company website as well as an online store under a subdomain where clients can purchase your products. In addition, you can have a forum where they can discuss the products and by using subdomains instead of subfolders you'll avoid any risk of all websites going down when you perform maintenance, or update one of the site scripts. Keeping your sites separated is more secure in the case of a script security breach.