A subdomain is part of a web address which is under the main domain name, for instance name.example.com. Actually, even in www.example.com the "www" element is a subdomain because the fully qualified domain name is just "example.com". Every subdomain can have its own website and records and can also be hosted using a different company if you need to use some feature that is not provided by your current service provider. A good example for using a subdomain is if you have a company site and an online store under a subdomain where clients can purchase your products. In addition, you can have a forum where they can talk about the products and by employing subdomains instead of subfolders you are going to avoid any probability of all sites going down when you perform maintenance, or update one of the website scripts. Keeping your websites separated is also less risky in case of a script security breach.