A subdomain is part of a web address that is under the main domain name, such as name.example.com. From a technical perspective, even in www.example.com the "www" part is a subdomain since the fully qualified domain name is only "example.com". Each subdomain can have its own site and records and can also be hosted using a different company if you want to use a certain feature that's not provided by your current service provider. One example for using a subdomain is if you have a business site along with an online store under a subdomain where clients can purchase your products. You can also have a forum in which they can discuss the products and by using subdomains rather than subfolders you will avoid any risk of all websites going down when you perform maintenance, or update one of the website scripts. Keeping your sites separated is also less risky in case there is a script security breach.