A subdomain is part of a web address that is under the main domain, such as name.example.com. Technically, even in www.example.com the "www" element is a subdomain since the fully qualified domain name is only "example.com". Every subdomain could have its own site and records and can even be hosted with a different provider if you want to use a feature that isn't offered by your current service provider. An illustration for using a subdomain is if you have a company website along with an online store under a subdomain where customers can purchase your products. You can even have a forum where they can comment on the products and by using subdomains as opposed to subfolders you'll avoid any probability of all websites going down when you perform maintenance, or update one of the site scripts. Keeping your sites separated is also less risky in the case of a script security breach.