A subdomain is part of a web address that's under the main domain, for instance name.example.com. From a technical perspective, even in www.example.com the "www" element is a subdomain because the fully qualified domain name is just "example.com". Each subdomain can have its own website and records and can even be hosted using a different company if you want to use some feature which isn't offered by your current provider. One example for using a subdomain is if you have a company site along with an online store under a subdomain where customers can acquire your products. You can also have a forum in which they can discuss the products and by using subdomains instead of subfolders you will avoid any chance of all sites going down if you perform maintenance, or update one of the website scripts. Keeping the sites separated is also less risky in case there is a script security breach.