Ajax, shorthand for Asynchronous JavaScript and XML, is a web development technique for creating interactive web applications. The intent is to make web pages feel more responsive by exchanging small amounts of data with the server behind the scenes, so that the entire web page does not have to be reloaded each time the user makes a change. This is meant to increase the web page's interactivity, speed, and usability.

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.

Ajax is only a name given to a set of tools that were previously existing.The main part is XMLHttpRequest, a class usable in JavaScript , that was implemented into Internet Explorer since the 4.0 version.

XMLHttpRequest in Synchronous mode which could have a negative effect specially on high load or slow website link, because the JavaScript engine is blocked until the request completes. That's why today we use AJAX and not SJAX, and the complexity of asynchronous programming is better handled today with the multitude of frameworks and techniques which make AJAX interfaces more fluid.

Asynchronous I/O solves this problem. When you make an asynchronous request it immediately returns - before it has finished. Sometime later when your request completes (it could be milliseconds or hours), Your program can then check on the state of the request to see if it completed successfully or if there was an error.