A subdomain is part of a web address that's under the main domain name, such as 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 single subdomain could have its own web site and records and can even be hosted through a different provider if you want to use a feature which isn't offered by your current provider. A good example for using a subdomain is if you have a business site as well as an online store under a subdomain where customers can purchase your products. You can also have a forum where they can discuss the products and by employing subdomains as opposed to subfolders you'll avoid any chance of all sites going down if you perform maintenance, or update one of the website scripts. Keeping your websites separated is also more secure in case of a script security breach.