A subdomain is a part of a web address that's under the main domain name, such as name.example.com. Technically, even in www.example.com the "www" element is a subdomain since the fully qualified domain name is just "example.com". Every single subdomain could have its own web site and records and can also be hosted through a different provider if you want to use some feature that is not provided by your current provider. An example for using a subdomain is if you have a business website as well as an online store under a subdomain where clients can purchase your products. Additionally, you can have a forum where they can discuss the products and by using subdomains rather than subfolders you'll avoid any risk of all websites going down if you perform maintenance, or update one of the site scripts. Keeping your sites separated is less risky in the event of a script security breach.