A server which holds and makes content available to the browser request from all around the internet via a special protocol calls such has html, xml, Javascript etc is known as web server. When a user of the internal opens up browser and type in an URL to reach a particular website, then essentially the computer or cell phone browser uses http or similar protocol to talk to the target web server and requests the desired content such as typing in https://www.pustak.org will open up home page of pustak.org.