A subdomain is part of a web address that is under the main domain, for example name.example.com. Technically, even in www.example.com the "www" part is a subdomain since 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 with a different provider if you want to use some feature that is not offered by your current provider. A good example for using a subdomain is if you have a business website and an online store under a subdomain where customers can buy your products. You can also have a forum where they can discuss the products and by employing subdomains rather than subfolders you will avoid any probability of all sites going down when you perform maintenance, or update one of the website scripts. Keeping your websites separated is less risky in case of a script security breach.