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 because the fully qualified domain name is only "example.com". Every single subdomain could have its own site and records and can even be hosted with a different provider if you wish to use a certain feature which isn't provided 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. Additionally, you can have a forum where they can talk about the products and by employing subdomains as an alternative to subfolders you are going to avoid any risk of all websites going down if you perform maintenance, or update one of the site scripts. Keeping your sites separated is more secure in the event of a script security breach.