A subdomain is a part of a web address which is under the main domain, for example name.example.com. Technically, even in www.example.com the "www" part is a subdomain because the fully qualified domain name is only "example.com". Each subdomain could have its own web site and records and can even be hosted through a different company if you wish to use a certain feature which is not offered by your current service 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 acquire your products. You can also have a forum in which they can discuss the products and by using subdomains as opposed to subfolders you will avoid any probability of all websites going down when you perform maintenance, or update one of the website scripts. Keeping your websites separated is also less risky in the event of a script security breach.