A subdomain is part of a web address that is under the main domain name, such as name.example.com. From a technical perspective, even in www.example.com the "www" element is a subdomain given that the fully qualified domain name is only "example.com". Each subdomain can have its own web site and records and can also be hosted with a different company if you want to use a feature which isn't provided by your current provider. One example for using a subdomain is if you have a company site and an online store under a subdomain where customers can acquire your products. Additionally, you can have a forum where they can talk about the products and by employing subdomains as opposed to subfolders you are going to avoid any probability of all sites going down if you perform maintenance, or update one of the website scripts. Keeping your sites separated is more secure in the case of a script security breach.