A subdomain is a part of a web address that is under the main domain, for instance name.example.com. From a technical perspective, even in www.example.com the "www" part is a subdomain because the fully qualified domain name is just "example.com". Each subdomain can have its own web site and records and can even be hosted with a different provider if you need to use a certain feature that is not offered 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. Additionally, you can have a forum in which they can talk about the products and by using subdomains rather than subfolders you are going to avoid any chance of all sites going down when you perform maintenance, or update one of the site scripts. Keeping the sites separated is also more secure in the case of a script security breach.