A subdomain is a part of a web address that is under the main domain name, such as name.example.com. From a technical perspective, even in www.example.com the "www" element is a subdomain because the fully qualified domain name is only "example.com". Every subdomain can have its own website and records and can even be hosted through a different provider if you wish to use a certain feature that's not offered by your current provider. A good example 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. Additionally, you can have a forum in which they can talk about the products and by employing subdomains rather than subfolders you are going to avoid any probability 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 the case of a script security breach.