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 since the fully qualified domain name is only "example.com". Every single subdomain can have its own website and records and can even be hosted with a different provider if you need to use a specific feature that's not provided by your current provider. A good example for using a subdomain is if you have a company site along with an online store under a subdomain where customers can purchase your products. You can even have a forum in which they can talk about the products and by employing subdomains as an alternative to subfolders you will avoid any risk of all sites going down if you perform maintenance, or update one of the website scripts. Keeping the websites separated is less risky in case there is a script security breach.