When it is finished the CGI can optionally prepare a message to send back to the browser. This response must be in the form of a full HTML page, including all of the structural tags, so the CGI must be able to generate such text. The CGI does not send the response page back directly to the broswer, but to the server that submitted the data. The POST method is different. After contact has been made, it sends the data directly to the CGI by putting them in some standard place in the CGI machine's memory.
Any reply is sent back, as before, via the browser.
Often it doesn't matter if you use GET or POST, since the data that reach the CGI are identical in either case. But sometimes it can. In general, POST is more flexible, so it is a good idea to use it. The format of form data When form data are sent, they are first put into a standard format which the server can recognize and use. Usually, this will be whatever the user typed in, but for a hidden field, it will be whatever HTML or JavaScript assigned as a value of the field.