A subdomain is part of a web address that is under the main domain, for instance name.example.com. Actually, even in www.example.com the "www" element is a subdomain since the fully qualified domain name is only "example.com". Each subdomain could have its own web site and records and can also be hosted with a different company if you need to use a feature that's not provided by your current service provider. One example for using a subdomain is if you have a company site along with an online store under a subdomain where clients can purchase your products. You can even have a forum where they can discuss the products and by employing subdomains as an alternative to subfolders you are going to avoid any chance of all sites going down if you perform maintenance, or update one of the website scripts. Keeping your sites separated is also less risky in the event of a script security breach.