Drupal 7 — Basic Ajax request (manual)

demo.js

/**
 * @file demo JS.
 * jQuery 1.5+
 */

(function( $ ) {

// On DOM ready
$(function() {

  var content = $("#block-system-main").find(".content")
    , link = $("#demo-ajax-link")
    , req

  link.click(function( e ) {
    e.preventDefault()

    req = $.ajax("/demo/ajax", {
      dataType: "json",
      type: "GET"
    })

    req.done(function( data, textStatus, jqXHR ) {
      content.append( data.data )
    })

    req.fail(function( jqXHR, textStatus, errorThrown ) {
      
    })

    req.always(function( jqXHR, textStatus ) {
      
    })

  })

})

})( jQuery );

demo.module

<?php

/**
 * @file
 * Demo module, Basic Ajax request (manual).
 */

/**
 * Implements hook_menu().
 */
function demo_menu() {
  return array(
    'demo/page' => array(
      'page callback' => 'demo_page_callback',
      'access callback' => TRUE,
      'type' => MENU_CALLBACK,
    ),
    'demo/ajax' => array(
      'page callback' => 'demo_ajax_callback',
      'access callback' => TRUE,
      'type' => MENU_CALLBACK,
      'delivery callback' => 'demo_ajax_delivery_callback',
    ),
  );
}

/**
 * Demo page callback.
 */
function demo_page_callback() {
  $path = drupal_get_path('module', 'demo');
  drupal_add_js($path . '/demo.js');
  
  $output = '<p>This is an ajax demo link: ';
  $output .= l('Click me ! ', 'demo/page', array('fragment' => 'nojs', 'attributes' => array('id' => array('demo-ajax-link'))));
  $output .= '</p>';

  return $output;
}

/**
 * Demo Ajax callback.
 */
function demo_ajax_callback() {
  $response = array(
    'status' => 1,
    'data' => 'Hello ! ',
  );

  return drupal_json_output($response);
}

/**
 * Demo Ajax delivery callback.
 */
function demo_ajax_delivery_callback($page_callback_result) {
  print $page_callback_result;
  ajax_footer();
}