PHP enables you to do a lot with web applications using an extremely thin client. One of its drawbacks, however, is the delay between page loads and its inability to get new data without submitting a form. AJAX gives you an additional communication channel with your PHP server, allowing you to send and receive data without reloading the page. This opens up a whole new world of possibilities and, when matched with JavaScript DOM manipulation, lets you create rich, interactive applications that remove the "click-and-wait" you have suffered through for years.