A subdomain is part of a web address that's under the main domain name, for instance name.example.com. Actually, even in www.example.com the "www" element is a subdomain given that the fully qualified domain name is just "example.com". Every single subdomain can have its own web site and records and can also be hosted through a different company if you wish to use some feature which isn't provided by your current provider. An illustration for using a subdomain is if you have a company site and an online store under a subdomain where clients can acquire your products. Additionally, you can have a forum where they can discuss the products and by employing subdomains as an alternative to subfolders you are going to avoid any probability of all sites going down when you perform maintenance, or update one of the site scripts. Keeping your websites separated is also less risky in case of a script security breach.