A subdomain is part of a web address which is under the main domain, such as name.example.com. Technically, even in www.example.com the "www" element is a subdomain since the fully qualified domain name is only "example.com". Each subdomain can have its own website and records and can also be hosted using a different company if you want to use a feature which isn't provided by your current provider. An example for using a subdomain is if you have a company website as well as an online store under a subdomain where clients can purchase your products. Additionally, you can have a forum in which they can comment on the products and by employing subdomains as an alternative to subfolders you are going to avoid any chance of all websites going down if you perform maintenance, or update one of the site scripts. Keeping your websites separated is also less risky in the case of a script security breach.