A subdomain is a part of a web address which is under the main domain, for example name.example.com. Actually, even in www.example.com the "www" element is a subdomain because the fully qualified domain name is only "example.com". Each subdomain could have its own web site and records and can even be hosted through a different provider if you wish to use a certain feature which is not offered by your current provider. One example for using a subdomain is if you have a company site along with an online store under a subdomain where customers can acquire your products. In addition, you can have a forum where they can discuss the products and by using subdomains as opposed to subfolders you are going to avoid any chance 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 case of a script security breach.