A subdomain is a part of a web address which is under the main domain, for instance name.example.com. Technically, even in www.example.com the "www" element is a subdomain because the fully qualified domain name is just "example.com". Every subdomain can have its own site and records and can even be hosted with a different company if you wish to use a specific feature that isn't provided by your current service provider. A good example for using a subdomain is if you have a business site along with an online store under a subdomain where customers can acquire your products. Additionally, you can 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 sites going down if you perform maintenance, or update one of the site scripts. Keeping your sites separated is more secure in the case of a script security breach.