A subdomain is a part of a web address which is 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 subdomain could have its own web site and records and can even be hosted through a different provider if you need to use a certain feature that isn't provided by your current service provider. An illustration for using a subdomain is if you have a business site as well as an online store under a subdomain where customers can acquire your products. You can also have a forum where they can talk about the products and by employing subdomains instead of subfolders you will avoid any chance of all websites going down when you perform maintenance, or update one of the site scripts. Keeping your websites separated is also less risky in the event of a script security breach.