A subdomain is part of a web address that is under the main domain, such as name.example.com. Actually, even in www.example.com the "www" part is a subdomain given that the fully qualified domain name is just "example.com". Every single subdomain can have its own website and records and can even be hosted through a different provider if you wish to use a certain feature that's not offered by your current provider. An illustration for using a subdomain is if you have a business site along with an online store under a subdomain where customers can buy your products. In addition, you can have a forum where they can comment on the products and by employing subdomains instead of subfolders you are going to avoid any probability of all websites going down when you perform maintenance, or update one of the site scripts. Keeping your sites separated is also less risky in the case of a script security breach.