A subdomain is 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 given that the fully qualified domain name is only "example.com". Every single subdomain can have its own website and records and can even be hosted using a different provider if you want to use a specific feature which is not provided by your current service 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 acquire your products. You can also have a forum in which they can comment on the products and by using subdomains as opposed to subfolders you are going to avoid any risk of all websites going down when you perform maintenance, or update one of the website scripts. Keeping your websites separated is also less risky in case there is a script security breach.