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 given that the fully qualified domain name is only "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 certain feature that's not provided by your current service provider. An example for using a subdomain is if you have a business website along with an online store under a subdomain where customers can buy your products. You can also have a forum where they can comment on the products and by using subdomains as opposed to subfolders you are going to avoid any probability of all sites going down if you perform maintenance, or update one of the website scripts. Keeping your websites separated is less risky in the event of a script security breach.