Making your users wait for anything that you present to them is never a good thing. Even if the wait is only a few milliseconds on a fast connection. We live in an 'immediate gratification' society where we expect things to work in a certain way and any deviation from that is seen as a poor implementation.
Waiting for page reloads, for example, while a form is executing or when you are waiting for a stock price come up. Synchronous web applications rely on client-server communication to be completed before results are returned from a form or other application. Asynchronous communication via Ajax let's JavaScript handle the client-server communication so that the user doesn't have to wait.
The idea behind Asynchronous JavaScript and XMl is the utilization of a request object in a JavaScript handle server side XMLHttpRequest's on the fly as the user is potentially doing other things. They don't have to wait for page reloads to interrupt their workflow.
Ajax, when done well (see Netflix, for example), also takes advantage of the DOM (Document Object Model) to swap out content on a page or to replace images in a gallery based on preference. All the while, the page remains visible to the user without reloading.
Ajax is also responsible for applications created by Google including Google Maps and Gmail.
There are some problems with Ajax that directly affect the user. Since the client-server communication is being handled by JavaScript, there are few indicators of the status of that communication. Since Ajax relies on ActiveX to handle the request, older browsers (pre-6.0) may have difficulty with some apps. Finally, since the client-server communication is happening behind the scenes, the use of the back button may be broken in some instances.
Overall though, Ajax is an exciting implementation of web technologies that have been around for a while: JavaScript and XML and the immediate page response is better than making users wait.
Al Lemieux
Senior Designer
SyberWorks, Inc.
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