A subdomain is part of a web address which is under the main domain, for example name.example.com. Actually, 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 website and records and can even be hosted through a different company if you want to use some feature that is not offered by your current provider. An example for using a subdomain is if you have a company website along with an online store under a subdomain where customers can purchase your products. In addition, you can have a forum where they can discuss the products and by using subdomains as an alternative to subfolders you are going to avoid any risk of all sites going down when you perform maintenance, or update one of the site scripts. Keeping the sites separated is less risky in case of a script security breach.
