A subdomain is part of a web address that's 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 single subdomain can have its own web site and records and can also be hosted using a different provider if you wish to use a specific feature that is not provided by your current service provider. An example for using a subdomain is if you have a business site along with an online store under a subdomain where customers can buy your products. You can even have a forum where they can talk about the products and by using subdomains instead of subfolders you'll avoid any risk of all websites going down if you perform maintenance, or update one of the site scripts. Keeping the websites separated is less risky in case there is a script security breach.