In these days all web2.0 apps make use of AJAX to save bandwidth, user time or to display data without refreshing the page. When you make an AJAX request the response could be plain text, Html or JSON, but if you want to make a cross-domain AJAX call you can’t because the browser security model dictates that XMLHttpRequest must have the same domain in order to communicate. This is where you need to use JSONP, this technique will make the AJAX call and after will inject the response in a script tag and will pass the data to a specified function.
Some JavaScript Libraries help you dealing with JSONP and in this article I will show you how to make cross-domain AJAX calls with MooTools and jQuery.