A subdomain is part of a web address which is under the main domain name, for example name.example.com. Actually, even in www.example.com the "www" element is a subdomain given that the fully qualified domain name is only "example.com". Every subdomain could have its own site and records and can even be hosted with a different provider if you wish to use a feature that 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 clients can buy your products. In addition, you can have a forum where they can comment on the products and by employing subdomains instead of subfolders you will avoid any risk of all sites going down if you perform maintenance, or update one of the website scripts. Keeping the websites separated is more secure in case there is a script security breach.