A subdomain is part of a web address which is under the main domain, for example name.example.com. Actually, even in www.example.com the "www" element is a subdomain because the fully qualified domain name is just "example.com". Every single subdomain can have its own website and records and can also be hosted with a different company if you wish to use a specific feature that is not provided by your current provider. A good example for using a subdomain is if you have a company website as well as an online store under a subdomain where customers can acquire your products. You can also have a forum where they can comment on the products and by employing subdomains as an alternative to subfolders you will avoid any probability of all websites going down if you perform maintenance, or update one of the website scripts. Keeping the sites separated is more secure in case of a script security breach.