A subdomain is a part of a web address which is under the main domain, for example name.example.com. Technically, even in www.example.com the "www" element is a subdomain because the fully qualified domain name is just "example.com". Each subdomain could have its own web site and records and can also be hosted through a different provider if you want to use a certain feature that is not offered by your current service provider. An 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. You can also have a forum in which they can discuss the products and by using subdomains instead of subfolders you are going to avoid any chance of all websites going down when you perform maintenance, or update one of the website scripts. Keeping the sites separated is more secure in the case of a script security breach.