A subdomain is part of a web address that is under the main domain name, for example name.example.com. Technically, even in www.example.com the "www" element is a subdomain since the fully qualified domain name is just "example.com". Every subdomain could have its own website and records and can also be hosted using a different provider if you wish to use a specific feature which is not provided by your current service provider. An example for using a subdomain is if you have a company website and an online store under a subdomain where clients can acquire your products. Additionally, you can have a forum where they can talk about the products and by employing subdomains rather than subfolders you are going to avoid any chance of all websites going down when you perform maintenance, or update one of the website scripts. Keeping the sites separated is also less risky in case of a script security breach.