A subdomain is part of a web address which is under the main domain name, such as 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". Every subdomain can have its own site and records and can also be hosted through a different provider if you wish to use some feature that isn't provided by your current provider. An illustration for using a subdomain is if you have a company site and an online store under a subdomain where customers can buy your products. You can also have a forum in which they can discuss the products and by using subdomains instead of subfolders you will avoid any risk of all sites going down if you perform maintenance, or update one of the website scripts. Keeping the websites separated is also less risky in the event of a script security breach.