A subdomain is a part of a web address which is under the main domain, for instance name.example.com. Actually, even in www.example.com the "www" part is a subdomain given that the fully qualified domain name is just "example.com". Every single subdomain can have its own web site and records and can even be hosted through a different provider if you want to use a certain feature which is not offered by your current 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. Additionally, you can have a forum where they can talk about the products and by employing subdomains rather than subfolders you will avoid any chance of all sites going down if you perform maintenance, or update one of the site scripts. Keeping your sites separated is less risky in case there is a script security breach.