A subdomain is a part of a web address that is under the main domain, for instance name.example.com. Technically, even in www.example.com the "www" element is a subdomain since 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 need to use a feature that is not provided by your current provider. A good example for using a subdomain is if you have a company website along with an online store under a subdomain where clients can purchase your products. You can also have a forum in which they can comment on the products and by using subdomains as an alternative to subfolders you will avoid any probability of all websites going down when you perform maintenance, or update one of the website scripts. Keeping the sites separated is also less risky in the event of a script security breach.