A subdomain is a part of a web address that is under the main domain name, for example name.example.com. From a technical perspective, even in www.example.com the "www" element is a subdomain since the fully qualified domain name is only "example.com". Every subdomain could have its own website and records and can also be hosted through a different provider if you wish to use a feature that isn't 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 acquire your products. You can even have a forum in which they can talk about 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 website scripts. Keeping the websites separated is less risky in case there is a script security breach.