A subdomain is a part of a web address which is under the main domain name, for instance name.example.com. From a technical perspective, even in www.example.com the "www" part is a subdomain given that 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 a specific feature that is not offered by your current service provider. An illustration for using a subdomain is if you have a business website along with an online store under a subdomain where clients can acquire your products. Additionally, you can have a forum in which they can discuss the products and by employing subdomains rather than subfolders you'll avoid any probability of all sites going down when you perform maintenance, or update one of the site scripts. Keeping the websites separated is also less risky in case of a script security breach.