What is HTTP (HyperText Transfer Protocol)?

HTTP is the protocol of the Web, by which Servers and Clients (typically
browsers) communicate.  An HTTP transaction comprises a Request sent by
the Client to the Server, and a Response returned from the Server to
the Client.
Every HTTP request and response includes a message header, describing
the message.   These are processed by the HTTPD, and may often be
mostly ignored by CGI applications (but see below).
A message body may also be included:
  1) A HEAD or GET request sends only a header.   Any form data is encoded 
     in an HTTP_QUERY_STRING header field, which is available to the CGI
     program as an environment variable QUERY_STRING.
  2) A POST request sends both header and body.   The body typically
     comprises data entered by a user in a form.
  3) A HEAD request does not expect a body in the response.
  4) A GET or POST request will accept a response with or without a body,
     according to the header.   The body of a response is typically an
     HTML document.