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" part is a subdomain because the fully qualified domain name is just "example.com". Each 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 that's not provided by your current service provider. One example for using a subdomain is if you have a company website along with an online store under a subdomain where customers can acquire your products. Additionally, you can have a forum where they can discuss the products and by using subdomains rather than subfolders you'll avoid any probability of all sites going down if you perform maintenance, or update one of the website scripts. Keeping your sites separated is more secure in case of a script security breach.