Ajax is not an acronym

I just wanted to re-iterate myself here again that Ajax is not a specific technology but a methodology, it's a way of working with set of technologies.

In the traditional web applications server pushes the entire web page after a user-initiation. In Ajax web applications apart of web page can be modified this increasing speed and usability of an application. Through the power of Ajax, the pages of your application can exchange small amounts of data with the server without going through a form submit.

Most client side technologies are as listed below:

• HTML or XHTML
• CSS
• DOM Scripting
• XMLHttpRequest

The XMLHttpRequest object is the glue that binds the server to the browser.

Here is excellent Diagram. I like diagrams they explain more than words.

1. The user generates an event, such as by clicking a button. This results in a JavaScript call.
2. An XMLHttpRequest object is created and configured with a request parameter that includes the ID of the component that generated the event and any value that the user might have entered.
3. The XMLHttpRequest object makes an asynchronous request to the web server. An object (such as a servlet or listener) receives the request, processes it, and stores any data in the request to the data store. In the case of Ajax-aware JavaServer Faces components, the object that processes the request is a PhaseListener object. We'll cover that more later in the document.
4. The object that processed the request returns an XML document containing any updates that need to go to the client.
5. The XMLHttpRequest object receives the XML data, processes it, and updates the HTML DOM representing the page with the new data.

The diagram and points are from this article