A subdomain is part of a web address that's 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 since the fully qualified domain name is only "example.com". Every single subdomain could have its own website and records and can even be hosted using a different company if you need to use some feature that isn't offered by your current provider. A good example for using a subdomain is if you have a company website as well as an online store under a subdomain where clients can purchase your products. In addition, you can have a forum where they can comment on the products and by using subdomains rather than subfolders you will avoid any probability 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 of a script security breach.