A subdomain is part of a web address which is under the main domain, for example name.example.com. Technically, even in www.example.com the "www" part is a subdomain because the fully qualified domain name is only "example.com". Every single subdomain could have its own web site and records and can even be hosted through a different provider if you need to use a certain feature which is not provided by your current service provider. An illustration for using a subdomain is if you have a business site and an online store under a subdomain where customers can buy your products. You can even have a forum where they can discuss the products and by employing subdomains rather than subfolders you will avoid any chance of all websites going down if you perform maintenance, or update one of the website scripts. Keeping the websites separated is less risky in the event of a script security breach.