A subdomain is a part of a web address which is under the main domain name, for example name.example.com. Technically, 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 website and records and can even be hosted through a different provider if you need to use some feature that is not offered by your current provider. An example for using a subdomain is if you have a business website and an online store under a subdomain where customers can acquire your products. You can also have a forum where they can talk about the products and by using subdomains as opposed to subfolders you will avoid any chance of all sites going down if you perform maintenance, or update one of the website scripts. Keeping your sites separated is also more secure in case there is a script security breach.