A subdomain is part of a web address that is under the main domain, for instance name.example.com. From a technical perspective, even in www.example.com the "www" element is a subdomain given that the fully qualified domain name is just "example.com". Every subdomain can have its own website and records and can even be hosted using a different provider if you need to use a feature that isn't provided by your current service provider. A good example for using a subdomain is if you have a company website and an online store under a subdomain where clients can buy your products. You can also have a forum where they can comment on the products and by employing subdomains as opposed to subfolders you will avoid any probability of all sites going down when you perform maintenance, or update one of the website scripts. Keeping the websites separated is also less risky in case of a script security breach.