what is AJAX?

I've been assigned to one of the big project these days, I am quite excited although I loooove my usual project with my best colleague.

As soon as I got Test cases from the leader, she said
"You have to be very careful to test pages with AJAX..." then I was like...SHOOOOOT I don't even know what is AJAX!!!

so decided to study with google prof.

AJAX is about better Internet-applications.
w3schools

Hmmm...Ok...still need more details
Wikipedia-Ajax

Getting understand...but still hmmmmm @ ______________ @
Finally found good description in JP site.
今更聞けないAjax

Hope I can test Ajax page with good knowledge and different scenarios next time.
Good luck