The title is fairly generic, so if you have found this, congratulations, your life is about to become far simpler. When working on or with ajax enabled web sites we usually run into the issue of request times, users need to be shown that something is happening in the background and visually we accomplish this with an Ajax Loader. You would have seen plenty of these by now, little animated gifs that give you a sense the web-page is loading something.
Ajax Plugins