A subdomain is a part of a web address which is under the main domain, for example name.example.com. From a technical perspective, even in www.example.com the "www" part is a subdomain because the fully qualified domain name is only "example.com". Every single subdomain can have its own web site and records and can even be hosted using a different provider if you want to use a feature which is not provided by your current service 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. You can also have a forum where they can discuss the products and by employing subdomains as opposed to subfolders you will avoid any probability of all websites going down when you perform maintenance, or update one of the website scripts. Keeping the websites separated is less risky in case of a script security breach.