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What is AJAX and what problem does it solve?

Ajax: It is a short for Asynchronous JavaScript and XML. It solves the problem of unnecessary data transfers and allows asynchronous processing and avoids unnecessary processing to be done by server.

Ajax is a set of client side technologies that allows asynchronous communication between client and web server. In synchronous communication, complete round trip happens with each request/response action event when small data of the page to be refreshed. Ajax has solved this problem of posting entire information every time through asynchronous communication. 

XmlHttpRequest is the basic fundamental behind Ajax. This allows browser to communicate with server without making post backs.