A subdomain is part of a web address that is under the main domain, for instance name.example.com. Technically, even in www.example.com the "www" part is a subdomain given that the fully qualified domain name is only "example.com". Every subdomain could have its own website and records and can even be hosted using a different company if you wish to use a certain feature which is not provided by your current service provider. One example for using a subdomain is if you have a business site as well as an online store under a subdomain where customers can acquire your products. In addition, you can have a forum where they can talk about the products and by employing subdomains instead of subfolders you'll avoid any risk of all websites going down if you perform maintenance, or update one of the site scripts. Keeping your websites separated is also more secure in case of a script security breach.