A subdomain is part of a web address that's under the main domain, for example name.example.com. From a technical perspective, even in www.example.com the "www" element is a subdomain because the fully qualified domain name is only "example.com". Every 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 that isn't offered by your current provider. An illustration for using a subdomain is if you have a business site as well as an online store under a subdomain where customers can buy your products. You can also have a forum where they can talk about the products and by using subdomains as an alternative to subfolders you are going to avoid any chance of all websites going down when you perform maintenance, or update one of the website scripts. Keeping the websites separated is more secure in case of a script security breach.