A subdomain is part of a web address which is under the main domain, such as name.example.com. Actually, even in www.example.com the "www" element is a subdomain because the fully qualified domain name is only "example.com". Each subdomain can have its own web site and records and can also be hosted through a different company if you need to use a specific feature that's 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 clients can buy your products. Additionally, you can have a forum in which they can talk about the products and by employing subdomains as an alternative to subfolders you'll avoid any risk of all sites going down if you perform maintenance, or update one of the site scripts. Keeping your websites separated is also more secure in case of a script security breach.