A subdomain is part of a web address that is under the main domain name, such as 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 can have its own web site and records and can also be hosted through a different provider if you need to use a specific feature that isn't provided by your current provider. One example for using a subdomain is if you have a company site and an online store under a subdomain where customers can purchase your products. You can also have a forum where they can talk about the products and by employing subdomains instead of subfolders you are going to avoid any risk of all sites going down if you perform maintenance, or update one of the site scripts. Keeping the sites separated is also more secure in the event of a script security breach.