HTTP Protocol Overview:

HTTP is the abbreviation of The HyperText Transfer Protocol. HTTP Version 1.1 is documented in RFC 2616. HTTP operates over TCP connections, usually to port 80. HTTP defines how messages are formatted and transmitted, and what actions Web servers and browsers should take in response to various commands. HTTP is called a stateless protocol because each command is executed independently, without any knowledge of the commands that came before it.

The simplest HTTP message is "GET url", to which the server replies by sending the named document. Most HTTP requests are GET requests. In addition to GET requests, clients can also send HEAD and POST requests, of which POSTs are the most important. Usually, GET asks to retrieve a document, POST passes form data to the server for use as input to some CGI program, and HEAD asks to retrieve only the HTTP response header for a document but not the document itself.

References:

Methods GET and POST in HTML forms - What's the difference?