A subdomain is a part of a web address that is under the main domain name, for example 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 only "example.com". Each subdomain could have its own site and records and can even be hosted through a different provider if you need to use some feature that's not offered by your current service 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 customers can buy your products. You can also have a forum in which they can talk about the products and by employing subdomains as opposed to subfolders you will avoid any chance 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 the case of a script security breach.