A subdomain is a part of a web address that's under the main domain name, for example name.example.com. Technically, 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 even be hosted through a different company if you want to use some feature which is not offered by your current service provider. An illustration for using a subdomain is if you have a company website and an online store under a subdomain where customers can buy your products. Additionally, you can have a forum in which they can comment on the products and by employing subdomains as an alternative to subfolders you are going to avoid any risk of all sites going down if you perform maintenance, or update one of the website scripts. Keeping your sites separated is less risky in case there is a script security breach.