A subdomain is part of a web address which is under the main domain name, for instance name.example.com. Technically, even in www.example.com the "www" element is a subdomain since the fully qualified domain name is only "example.com". Every single subdomain can have its own web site and records and can even be hosted through a different provider if you wish to use a certain feature that is not provided 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 clients can buy your products. Additionally, you can have a forum in which they can talk about the products and by using 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 website scripts. Keeping the sites separated is more secure in the event of a script security breach.