A subdomain is a 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" part is a subdomain given that the fully qualified domain name is only "example.com". Each subdomain could have its own site and records and can also be hosted with a different provider if you want to use a specific feature that is not provided by your current provider. An illustration for using a subdomain is if you have a company website and an online store under a subdomain where customers can buy your products. In addition, you can have a forum in which they can comment on the products and by using subdomains rather than subfolders you'll avoid any chance of all websites going down when 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.