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" element is a subdomain given that the fully qualified domain name is just "example.com". Every subdomain could have its own web site and records and can even be hosted using a different provider if you need to use some feature that is not offered by your current service provider. A good example for using a subdomain is if you have a business site and an online store under a subdomain where clients can acquire your products. In addition, you can have a forum in which they can comment on the products and by employing subdomains as opposed to subfolders you are going to avoid any probability of all websites going down if you perform maintenance, or update one of the site scripts. Keeping the sites separated is less risky in case of a script security breach.