AJAX is a set of web development techniques that uses several client-side web technologies to make interactive web apps that can handle user queries in real time. AJAX achieves this by using HTML and CSS for visual rpesentation, XML or JSON for interchange of the data as well as XSLT for manipulation, the Document Object Model (DOM) for dynamic data display, XMLHttpRequest. for asynchronous communication and JavaScript to unify these technologies.
AJAX is the key to allowing content on the web page to be updated instantly, doing so as soon as a visitor performs a particular action. This is different from an HTTP request, where the entire page is reloaded. Because AJAX is heavily reliant on XMLHttpRequest, the first applications to use it would only work with the Internet Explorer browser, but every modern web browser fully supports AJAX.