A subdomain is a part of a web address that's under the main domain name, such as 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 web site and records and can also be hosted through a different company if you wish to use a specific feature that's not offered by your current provider. An illustration for using a subdomain is if you have a company site as well as an online store under a subdomain where clients can purchase your products. You can even have a forum in which they can discuss the products and by employing subdomains rather than subfolders you will avoid any probability of all sites going down when you perform maintenance, or update one of the website scripts. Keeping the websites separated is more secure in case of a script security breach.