AJAX stands for Asynchronous JavaScript And XML.

AJAX is a type of programming made popular in 2005 by Google (with Google Suggest).

AJAX is not a new programming language, but a new way to use existing standards.

With AJAX you can create better, faster, and more user-friendly web applications.

AJAX is based on JavaScript and HTTP requests.

I’m going to tell you new piece of Ajax information.
Ok what’s Ajax? - Asynchronous JavaScript + XML, new great technology which closely related with conception of web2. If you saw amazing products of Google(gmail, google maps) you could notice that this applications is very  similar with usual desktop applications it’s Ajax. The main point of using it is the great usability, speed of interactions and a big traffic’s economy. And your applications don’t need postbacks!!!
But it has one disadvantage you need to write a lot of client side code so you should be careful to manage with it.

A lead to a great posting Rasmus' 30 second Ajax tutorial was too good a treat not to share around. Sometimes we all want something very simple to build a thorough understanding of the mechanics of a new technique before we dive into the deeper water beyond. Now, if you are into ASP.NET and not PHP you might like to take a look at my version of this ultra-simple introduction to Ajax with sincere thanks to the original author. Now most simple introductions to Ajax using ASP.NET seem to quickly become complex - introducing libraries and DLLs and helper functions just about as soon as the first paragraph is over. This is because they all seem to miss out the fact that you have to work around the ASP.Net functionality to get thing working in a nice simple manner. I will try and take you through the steps that explain the problem and thus make the solutions seem much more obvious.

Ajax stands for Asynchronous JavaScript and XML. In a nutshell, it is the use of the nonstandard XMLHttpRequest object to communicate with server-side scripts. It can send as well as receive information in a variety of formats, including XML, HTML, and even text files. Ajax’s most appealing characteristic, however, is its “asynchronous” nature, which means it can do all of this without having to refresh the page. This allows you to update portions of a page based upon user events.

Get an introduction to ECMAScript for XML (E4X), a simple extension to JavaScript that makes XML scripting very simple. In this paper, the authors demonstrate a Web programming model called Asynchronous JavaScript and XML (AJAX) and show you how some new XML extensions to JavaScript can make it very simple.