A subdomain is 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" element is a subdomain given that the fully qualified domain name is just "example.com". Every single subdomain could have its own website and records and can also be hosted through a different company if you want to use a feature that 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 where they can discuss the products and by using subdomains as opposed to subfolders you are going to avoid any risk of all sites going down when you perform maintenance, or update one of the site scripts. Keeping your sites separated is less risky in the case of a script security breach.