A subdomain is a part of a web address that's under the main domain name, such as name.example.com. Actually, even in www.example.com the "www" part is a subdomain because the fully qualified domain name is just "example.com". Every subdomain can have its own web site and records and can even be hosted with a different company if you want to use a certain feature that is not provided by your current provider. An example for using a subdomain is if you have a company website along with an online store under a subdomain where customers can buy your products. You can even have a forum where they can discuss the products and by using subdomains instead of subfolders you will avoid any chance of all sites going down if you perform maintenance, or update one of the site scripts. Keeping your websites separated is less risky in case there is a script security breach.