A subdomain is part of a web address which is under the main domain, for example name.example.com. From a technical perspective, even in www.example.com the "www" part is a subdomain because the fully qualified domain name is just "example.com". Every single subdomain could have its own site and records and can also be hosted through a different provider if you need to use a specific feature that's not provided by your current provider. One example for using a subdomain is if you have a business site and 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 employing subdomains as an alternative to subfolders you will avoid any chance 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.