A subdomain is a part of a web address which is under the main domain name, 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". Every subdomain can have its own website and records and can even be hosted with a different company if you want to use some feature that's not offered by your current provider. A good example for using a subdomain is if you have a company site as well as an online store under a subdomain where clients can buy your products. In addition, you can have a forum where they can comment on the products and by using subdomains as an alternative to subfolders you will avoid any chance of all sites going down if you perform maintenance, or update one of the website scripts. Keeping your sites separated is more secure in the case of a script security breach.