A subdomain is part of a web address that is under the main domain name, such as name.example.com. Technically, even in www.example.com the "www" element is a subdomain since the fully qualified domain name is only "example.com". Each subdomain could have its own site and records and can even be hosted with a different company if you want to use a feature which is not provided by your current provider. A good example for using a subdomain is if you have a company website as well as an online store under a subdomain where customers can acquire your products. You can even have a forum where they can comment on the products and by using subdomains as an alternative to subfolders you are going to 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 event of a script security breach.