A subdomain is part of a web address that's under the main domain name, for example 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". Every single subdomain can have its own website and records and can even be hosted through a different provider if you need to use some feature which isn't provided by your current service provider. A good example for using a subdomain is if you have a business site as well as an online store under a subdomain where clients can acquire your products. You can also have a forum where they can talk about the products and by employing subdomains as opposed to subfolders you'll avoid any probability of all websites going down when you perform maintenance, or update one of the website scripts. Keeping the sites separated is less risky in the event of a script security breach.