A subdomain is part of a web address that's under the main domain name, for instance name.example.com. Actually, even in www.example.com the "www" part is a subdomain given that the fully qualified domain name is only "example.com". Every single subdomain could have its own website and records and can even be hosted through a different company if you want to use a certain feature that's not offered by your current provider. One example for using a subdomain is if you have a business site along with an online store under a subdomain where clients can purchase your products. In addition, you can have a forum in which they can discuss the products and by employing subdomains rather than subfolders you will avoid any probability of all websites going down when you perform maintenance, or update one of the site scripts. Keeping your websites separated is also less risky in the case of a script security breach.