A subdomain is part of a web address that is under the main domain name, for instance name.example.com. Actually, even in www.example.com the "www" element is a subdomain since the fully qualified domain name is just "example.com". Every subdomain could have its own web site and records and can even be hosted through a different provider if you need to use a specific feature that isn't provided by your current provider. A good example for using a subdomain is if you have a business website and an online store under a subdomain where clients can buy your products. In addition, you can have a forum where they can discuss the products and by employing subdomains as an alternative to subfolders you'll avoid any chance of all sites going down when you perform maintenance, or update one of the website scripts. Keeping your websites separated is also less risky in the case of a script security breach.