HTTP is the standard protocol for transferring data across the Internet; it includes the header (where information about the payload is sent) and the payload (where the response itself is sent). Every HTTP transaction consists of a request from the client (web browser) to a web server and a response from the web server to the client.
There are many applications for HTTP. For the purpose of this text, we will just focus on web browser requests and HTML responses. It is important to understand the HTTP transaction in order to see the effect that it can have on the ranking of each document.