A subdomain is part of a web address that is under the main domain, such as name.example.com. From a technical perspective, even in www.example.com the "www" part is a subdomain since the fully qualified domain name is just "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 certain feature which is not provided by your current provider. An illustration for using a subdomain is if you have a business site along with an online store under a subdomain where clients can purchase your products. You can even have a forum where they can talk about the products and by employing subdomains instead of subfolders you are going to avoid any probability 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.