At the heart of the world wide web is the Hyper-Text Transfer Protocol (HTTP). This is a protocol defining how HTTP servers (which host web pages) interact with HTTP clients (which display web pages).
It starts with a request initiated from the web browser (the client). This request is sent over the Internet using the TCP protocol to a web server. Once the web server receives the request, it must decide the appropriate response - ideally sending the requested resource back to the browser to be displayed. The following diagram displays this typical request-response pattern.
This HTTP request-response pattern is at the core of how all web applications communicate. Even those that use websockets begin with an HTTP request.