A subdomain is a part of a web address that's under the main domain, for example name.example.com. From a technical perspective, even in www.example.com the "www" element is a subdomain given that the fully qualified domain name is just "example.com". Every single subdomain could have its own web site and records and can even be hosted with a different provider if you want to use a specific feature that isn't provided 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 purchase your products. You can even have a forum where they can talk about the products and by using subdomains as an alternative to subfolders you'll avoid any risk of all sites going down when you perform maintenance, or update one of the site scripts. Keeping your websites separated is less risky in the event of a script security breach.