A subdomain is part of a web address that's 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 only "example.com". Every single subdomain can have its own site and records and can also be hosted using a different provider if you wish to use a feature which isn't offered by your current provider. An illustration for using a subdomain is if you have a business website and an online store under a subdomain where customers can buy your products. In addition, you can have a forum where they can talk about the products and by using subdomains rather than subfolders you are going to avoid any chance of all sites going down when you perform maintenance, or update one of the website scripts. Keeping the sites separated is also less risky in the event of a script security breach.