A subdomain is part of a web address that's under the main domain, such as name.example.com. Technically, even in www.example.com the "www" element is a subdomain since the fully qualified domain name is just "example.com". Every subdomain can have its own website and records and can even be hosted using a different provider if you need to use a specific feature that is not offered by your current provider. A good example for using a subdomain is if you have a business site as well as an online store under a subdomain where customers can acquire your products. In addition, you can have a forum in which they can comment on the products and by using subdomains as opposed to subfolders you'll avoid any probability of all sites going down when you perform maintenance, or update one of the website scripts. Keeping the sites separated is more secure in case of a script security breach.