It has been available ever since Internet Explorer 5.5 was released in July 2000, but not fully discovered before people started to talk about AJAX and Web 2.0 in 2005. What is that ?

The "core" and defining element of Ajax is the XMLHttpRequest object, which gives browsers the ability to make dynamic and asynchronous data requests without having to unload and reload a page. Given XMLHttpRequest can eliminate the need for page refreshes, other technologies become more prominently used and highlighted with this development approach