A subdomain is a part of a web address which is under the main domain, 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 with a different company if you wish to use a feature that's not offered by your current provider. A good example for using a subdomain is if you have a business site along with an online store under a subdomain where customers can purchase your products. You can also have a forum where they can talk about the products and by employing subdomains rather than subfolders you'll avoid any probability of all sites going down if you perform maintenance, or update one of the website scripts. Keeping your websites separated is also more secure in the event of a script security breach.