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