A subdomain is a part of a web address that is under the main domain, for example name.example.com. From a technical perspective, 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 website and records and can even be hosted through a different company if you want to use a specific feature that isn't provided by your current provider. An example for using a subdomain is if you have a business website as well as an online store under a subdomain where customers can acquire your products. Additionally, you can have a forum in which they can comment on the products and by employing subdomains as opposed to subfolders you are going to avoid any risk of all websites going down if you perform maintenance, or update one of the website scripts. Keeping the websites separated is also more secure in the event of a script security breach.