A subdomain is part of a web address that's under the main domain, for example name.example.com. From a technical perspective, even in www.example.com the "www" element is a subdomain because the fully qualified domain name is only "example.com". Every subdomain could have its own website and records and can even be hosted with a different provider if you need to use a certain feature that's not provided by your current service provider. A good example for using a subdomain is if you have a company site as well as an online store under a subdomain where customers can purchase your products. In addition, you can have a forum where they can comment on the products and by using subdomains as an alternative to subfolders you are going to avoid any risk of all sites going down when you perform maintenance, or update one of the website scripts. Keeping the sites separated is also less risky in case there is a script security breach.