The library comes to us via Jeff Veen, who offers some excellent perspective on designing for the subtlety that's possible with these new techniques:
ot all that long ago interaction on the Web was a lot simpler -- you clicked a link, the page you were on disappeared, you waited a bit, a new one appeared. No feedback was really necessary. Clearly, something had occured. Whether it was what you were expecting was another matter entirely. From an interaction point of view, there was no question that your action had a reaction from the system.
Enter Ajax. Now, I have the option of requesting bits of data whenever I need it, and updating just parts of a page that has already been rendered. No longer am I required to refresh an entire page every time a user clicks a link or submits data. With Ajax, I can chose.
For more of Jeff's thoughts, you can check out his essay which we've reprinted in the Professional Network articles section, Why Content Management Fails.