A subdomain is part of a web address that's under the main domain name, such as name.example.com. From a technical perspective, even in www.example.com the "www" part is a subdomain given that the fully qualified domain name is only "example.com". Every subdomain can have its own web site and records and can even be hosted through a different provider if you need to use a certain feature that's not provided by your current 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 buy your products. You can even have a forum where they can talk about the products and by employing subdomains rather than subfolders you will avoid any risk of all websites going down when you perform maintenance, or update one of the website scripts. Keeping your sites separated is less risky in case there is a script security breach.