A subdomain is a part of a web address that's under the main domain name, for instance 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 can have its own web site and records and can also be hosted with a different provider 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 site along with an online store under a subdomain where clients can purchase your products. Additionally, you can have a forum where they can discuss the products and by using subdomains as opposed to subfolders you are going to avoid any chance of all sites going down when you perform maintenance, or update one of the site scripts. Keeping your sites separated is also less risky in case of a script security breach.