A subdomain is a part of a web address that's under the main domain name, for instance name.example.com. From a technical perspective, even in www.example.com the "www" part is a subdomain given that the fully qualified domain name is only "example.com". Every single subdomain can have its own site and records and can even be hosted with a different provider if you wish to use a certain feature that is not offered by your current service provider. One example for using a subdomain is if you have a business site and an online store under a subdomain where customers can purchase your products. You can even have a forum in which they can discuss the products and by employing subdomains instead of subfolders you are going to avoid any risk of all websites going down when you perform maintenance, or update one of the website scripts. Keeping your websites separated is more secure in case of a script security breach.