A subdomain is a part of a web address which is under the main domain, for example name.example.com. Technically, even in www.example.com the "www" element is a subdomain because the fully qualified domain name is only "example.com". Each subdomain can have its own website and records and can also be hosted through a different company if you want to use a specific feature which isn't provided 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 where they can talk about the products and by employing subdomains as opposed to subfolders you will avoid any chance of all websites going down if you perform maintenance, or update one of the site scripts. Keeping the websites separated is also less risky in case of a script security breach.