What is “Ajax”?

Ajax is actually a family of technologies that have been available for years. The means to make requests to the server using only JavaScript were built into Internet Explorer 5.5, but the possibilities of the technology were overlooked. It was only in 2005 that the techniques were rediscovered and used, notably to excellent effect in Google’s » GMail web application.
The term Ajax, which stands for “Asynchronous JavaScript and XML”, was first coined by Jesse James Garrett in his somewhat infamous article, » Ajax: A New Approach to Web Applications.

So let’s take each of those parts in isolation. Ajax is:

Asynchronous: This means that when you send a request, you wait for the response to come back, but are free to do other things while you wait. The response probably won’t come back immediately, so you set up a function that will wait for the response to be sent back by the server, and react to it once that happens.

JavaScript: JavaScript is used to make a request to the server. Once the response is returned by the server, you will generally use some more JavaScript to modify the current page’s document object model in some way to show the user that the submission went through successfully.

XML: The data that you receive back from the server will often be packaged up as a snippet of XML, so that it can be easily processed with JavaScript. This data can be anything you want, and as long as you want.

There’s nothing really new about what is happening here. We’re requesting a file (which will often be a server-side script, coded in something like PHP), and receiving a page as the response. This is how the web works already — the only difference is that now we can make these requests from JavaScript.