A subdomain is a part of a web address which is under the main domain name, for instance 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 only "example.com". Every single subdomain can have its own site and records and can even be hosted using a different provider if you want to use a specific feature that's not offered by your current provider. An example for using a subdomain is if you have a business site as well as an online store under a subdomain where customers can purchase your products. In addition, you can have a forum where they can talk about the products and by using subdomains as opposed to subfolders you'll avoid any probability of all websites going down when you perform maintenance, or update one of the website scripts. Keeping your websites separated is also less risky in the case of a script security breach.