My friends asking me what is AJAX ? Football club from netherland ? Yes it is. But we are talking about AJAX in internet. I Search in google and found this result :
A scripting technique for silently loading new data from the server. Although AJAX scripts commonly use the soon to be standardized XMLHttpRequest object, they could also use a hidden iframe or frame. An AJAX script is useless by itself. It also requires a DOM Scripting component to embed the received data in the document.
the second result that i think really good explanation of AJAX is :