AJAX INTRO

INTRODUCTION TO AJAX
Ajax (Asynchronous JavaScript + XML) is a recent name coined by Jesse James Garrett of Adaptive Path. Ajax isn’t a new technology. It’s a mix of several old technologies coming together in powerful new ways.

AJAX COMPARED TO CLUNKY WEB INTERFACES

The classic web application model works like this: Most user actions in the interface trigger an HTTP request back to a web server. The server does some processing — retrieving data, talking to various legacy systems — and then returns an HTML page to the client.
This approach makes a lot of technical sense, but it doesn’t make for a great user experience. While the server is doing its thing, what’s the user doing? That’s right, waiting. And at every step in a task, the user waits some more.

Courtesy: http://www.adaptivepath.com/ideas/essays/archives/000385.php

An Ajax application eliminates the start-stop-start-stop nature of interaction on the Web by introducing an intermediary — an Ajax engine — between the user and the server. It seems like adding a layer to the application would make it less responsive, but the opposite is true.
Instead of loading a webpage, at the start of the session, the browser loads an Ajax engine — written in JavaScript and usually tucked away in a hidden frame. This engine is responsible for both rendering the interface the user sees and communicating with the server on the user’s behalf. The Ajax engine allows the user’s interaction with the application to happen asynchronously — independent of communication with the server. So the user is never staring at a blank browser window and an hourglass icon, waiting around for the server to do something.

When you write an application today, you have two basic choices:
Desktop applications
Web applications
Desktop applications are usually pretty fast (they're running on your computer; you're not waiting on an Internet connection), have great user interfaces (usually interacting with your operating system), and are incredibly dynamic. You can click, point, type, pull up menus and sub-menus, and cruise around, with almost no waiting around.
On the other hand, Web applications are usually up-to-the-second current and they provide services you could never get on your desktop (think about Amazon.com and eBay). However, with the power of the Web comes waiting -- waiting for a server to respond, waiting for a screen to refresh, and waiting for a request to come back and generate a new page.

Ajax attempts to bridge the gap between the functionality and interactivity of a desktop application and the always-updated Web application. You can use dynamic user interfaces and fancier controls like you'd find on a desktop application, but it's available to you on a Web application.

TECHNOLOGIES INVOLVED IN AJAX
CSS offers a way of defining reusable visual styles for web page elements. HTML is used to build Web forms and identify fields for use in the rest of your application.
JavaScript code is the core code running Ajax applications and it helps facilitate communication with server applications.
DHTML, or Dynamic HTML(combination of CSS,DOM and JavaScript), helps you update your forms dynamically. You'll use div, span, and other dynamic HTML elements to mark up your HTML.
DOM, the Document Object Model, will be used (through JavaScript code) to work with both the structure of your HTML and (in some cases) XML returned from the server.
XMLHttpRequest object is the heart of AJAX. It allows web programmers to retrieve data from the web server as a background activity. The data format is typically XML but it works well with any text-based data.