A subdomain is a part of a web address that is under the main domain, for example name.example.com. From a technical perspective, even in www.example.com the "www" element is a subdomain since the fully qualified domain name is just "example.com". Every single subdomain can have its own web site and records and can even be hosted through a different company if you wish to use some feature which isn't provided by your current provider. One example for using a subdomain is if you have a company website along with an online store under a subdomain where clients can buy your products. Additionally, you can have a forum where they can discuss the products and by using subdomains as opposed to subfolders you are going to avoid any risk of all sites going down when you perform maintenance, or update one of the site scripts. Keeping your sites separated is also more secure in the event of a script security breach.