A subdomain is a part of a web address which is under the main domain, for instance subdomain.domainname.com. From a technical perspective, even in www.domainname.com the “www” element is a subdomain given that the fully qualified domain name is only “domainname.com”.
Each subdomain can have its own website and records and can even be hosted with a different provider if you need to use some feature that’s not provided by your current provider.
One example for using a subdomain is if you have a business site along with an online store under a subdomain where customers can buy your products. In addition, you can have a forum where they can discuss the products and by using subdomains as an alternative to subfolders you are going to avoid any risk of all sites going down if you perform maintenance, or update one of the site scripts. Keeping your sites separated is also more secure in the event of a script security breach.
What Is A Subdomain?