A subdomain is a part of a web address which is under the main domain, such as name.example.com. Technically, even in www.example.com the "www" element is a subdomain given that the fully qualified domain name is only "example.com". Every subdomain can have its own web site and records and can even be hosted through a different provider if you wish to use a certain feature which is not offered by your current service provider. An example for using a subdomain is if you have a business website along with an online store under a subdomain where clients can buy your products. You can also have a forum where they can comment on the products and by employing subdomains rather than subfolders you'll avoid any risk of all sites going down if you perform maintenance, or update one of the website scripts. Keeping your websites separated is also more secure in case there is a script security breach.