A subdomain is a part of a web address which is under the main domain name, for instance name.example.com. Actually, even in www.example.com the "www" part is a subdomain because the fully qualified domain name is only "example.com". Every subdomain could have its own site and records and can also be hosted using a different company if you want to use a certain feature that is not provided by your current provider. One example for using a subdomain is if you have a business site and an online store under a subdomain where customers can acquire your products. You can also have a forum in which they can discuss the products and by using subdomains rather than subfolders you will avoid any probability of all websites going down if you perform maintenance, or update one of the website scripts. Keeping your websites separated is also less risky in the event of a script security breach.