A subdomain is a part of a web address that's under the main domain name, for example 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 also be hosted with a different provider if you wish to use a certain feature which is not provided by your current provider. An example for using a subdomain is if you have a company site as well as an online store under a subdomain where customers can acquire your products. Additionally, you can have a forum in which they can talk about the products and by using subdomains instead of subfolders you will avoid any chance of all websites going down if you perform maintenance, or update one of the site scripts. Keeping the websites separated is also less risky in case of a script security breach.