AJAX is not a technology.
It is really several technologies, each flourishing in its own right, coming together in powerful new ways.
AJAX incorporates:
Dynamic display and interaction using the Document Object Model, a standard way for accessing and manipulating HTML documents.
Data interchange and manipulation using XML and XSLT.
Asynchronous data retrieval using XMLHttpRequest.
By using the XMLHttpRequest object, a web developer can change a page with data from the server after the page has loaded.
The AJAX example below shows how a web page can communicate with a web server online as a user enters data into a standard HTML form:
Other AJAX examples include
Google Suggest:
Watch the way the suggested terms update as you type, almost instantly.
Google Maps:
Use your cursor to grab the map and scroll around a bit.
Again, everything happens almost instantly.
The biggest challenges in creating AJAX applications are not technical.
The core AJAX technologies are mature, stable, and well understood.
Instead, the challenges are for the designers of these applications:
to forget what we think we know about the limitations of the Web, and begin to imagine a wider, richer range of possibilities.