AJAX is a technique for creating fast and dynamic web pages.
It is not a new programming language, but a new way to use existing standards.
Classically we have to reload the entire page, if some content has to change.
Ajax is used on the client-side. It can used to prevent modifying with the display and behaviour of an entire loaded page. It thereby helps immensely improve the web page performance.
Along with text and variables, JSON is often used nowadays, as a format for input and output data for AJAX calls.
Testing & Debugging
Rather than working in isolation, Ajax combines into a group of interrelated Web development techniques/technologies.
AJAX applications are browser and platform independent.