A subdomain is a part of a web address that's under the main domain, such as name.example.com. Actually, even in www.example.com the "www" part is a subdomain because the fully qualified domain name is just "example.com". Every single subdomain can have its own website and records and can also be hosted through a different provider if you wish to use some feature that is not offered by your current service provider. An illustration for using a subdomain is if you have a company website and an online store under a subdomain where customers can acquire your products. You can even have a forum where they can discuss the products and by employing subdomains instead of subfolders you'll avoid any probability of all websites going down if you perform maintenance, or update one of the website scripts. Keeping the websites separated is more secure in the case of a script security breach.