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Ajax Introduction
AJAX stands for Asynchronous JavaScript and XML.It is the use of the  XMLHttpRequest object to communicate with servers. The Ajax was publicly used on 18 February 2005 by Jesse James Garrett.  
Mainly we are creating interactive web applications, based on techniques used on Google pages. [Google search, while we are typing some thing in Google search box it will show you some suggestions automatically that is Ajax.]

It is a group of inter-related technologies like JavaScript, XML, HTML, CSS etc.

AJAX is a web browser technology independent of web server software.

A user can continue to use the application while the client program requests information from the server in the background.

Ajax Best Examples :

When you start typing in Google's search box, a JavaScript sends the letters off to a server and the server returns a list of suggestions without reload that page.