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 just "example.com". Every single subdomain could have its own site and records and can also be hosted with a different company if you need to use some feature which is not provided by your current provider. A good 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. In addition, you can have a forum where they can comment on the products and by using subdomains as opposed to subfolders you'll avoid any probability of all websites going down if you perform maintenance, or update one of the website scripts. Keeping the sites separated is more secure in the event of a script security breach.