Asynchronous JavaScript and XML, is a web development
technique for 
creating interactive web applications.

Like DHTML, LAMP and SPA, Ajax is not a technology in itself, 
but a term that refers to the use of a group of technologies.

AJAX (Asynchronous JavaScript and XML) is a newly coined 
term for two powerful browser features that have been 
around for years, but were overlooked by many web 
developers until recently when applications such as Gmail, 
Google Suggest, and Google Maps hit the streets. 

Asynchronous JavaScript and XML, or Ajax (pronounced "Aye-
Jacks"), is a web development technique for creating 
interactive web applications using a combination of XHTML 
(or HTML) and CSS for marking up and styling information. 
(XML is commonly used, although any format will work, 
including preformatted HTML, plain text, JSON and even 
EBML). 
The Document Object Model manipulated through JavaScript to 
dynamically display and interact with the information 
presented 
The XMLHttpRequest object to exchange data asynchronously 
with the web server. In some Ajax frameworks and in some 
situations, an IFrame object is used instead of the 
XMLHttpRequest object to exchange data with the web server. 
Like DHTML, LAMP, or SPA, Ajax is not a technology in 
itself, but a term that refers to the use of a group of 
technologies together. In fact, derivative/composite 
technologies based substantially upon Ajax, such as AFLAX, 
are already appearing. 
Ajax applications are mostly executed on the user's 
computer; they can perform a number of tasks without their 
performance being limited by the network. This permits the 
development of interactive applications, in particular 
reactive and rich graphic user interfaces. 
Ajax applications target a well-documented platform, 
implemented by all major browsers on most existing 
platforms. While it is uncertain that this compatibility 
will resist the advent of the next generations of browsers 
(in particular, Firefox), at the moment, Ajax applications 
are effectively cross-platform. 
While the Ajax platform is more restricted than the Java 
platform, current Ajax applications effectively fill part 
of the one-time niche of Java applets: extending the 
browser with portable, lightweight mini-applications. 

Ajax isn't a technology. It's really several technologies, 
each flourishing in its own right, coming together in 
powerful new ways. Ajax incorporates: 
* standards-based presentation using XHTML and CSS; 
* dynamic display and interaction using the Document Object 
Model; 
* data interchange and manipulation using XML and XSLT; * 
asynchronous data retrieval using XMLHttpRequest; 
* and JavaScript binding everything together.