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 could have its own website and records and can also be hosted using a different provider if you want to use a feature that's not provided 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 acquire your products. Additionally, you can have a forum where they can discuss the products and by using subdomains rather than subfolders you are going to avoid any probability of all sites going down if you perform maintenance, or update one of the website scripts. Keeping your sites separated is more secure in the event of a script security breach.