Ajax is an acronym for Asynchronous Javascript and XML. It is basically a cross-platform mechanism which allows you to make fast, rich and responsive web pages. Using AJAX a coder can avoid reloading the web page when a part of it is changing because of the user interaction.

So the AJAX engine which runs within the context of the browser handles basic tasks of validation and oter changes while the script dynamically retrieve some data from the application. This data transfer happens from browser to application in the background and so known as ‘Asynchronous mode of execution’.   More detail...