A subdomain is part of a web address which is under the main domain name, for example name.example.com. From a technical perspective, even in www.example.com the "www" element is a subdomain since the fully qualified domain name is just "example.com". Each subdomain could have its own website and records and can even be hosted through a different company if you need to use a feature that's not offered by your current service provider. One example for using a subdomain is if you have a business website as well as an online store under a subdomain where clients can buy your products. You can even have a forum where they can comment on the products and by using subdomains rather than subfolders you are going to avoid any probability of all sites going down when you perform maintenance, or update one of the website scripts. Keeping your sites separated is also more secure in the event of a script security breach.