There has been a lot of buzz about this "AJAX" technology among web designer types. AJAX is a relatively new method of live communication between a web browser and a web server. AJAX is implemented with a client side scripting language, and a server side scripting language.
In my implementations of AJAX, I use JavaScript and PHP. So far I've only created a few trivial examples, but I hope to soon create something a bit more substantial.
My current AJAX projects are:
The chat room on this site. Messages are sent and retrieved as needed, avoiding wasteful page refreshes.
The counter that displays how long you have been viewing this site is another simple AJAX trick. It is a JavaScript that reports timings to the server, which tracks timing values across several pages.
If you click on this site a few times, you may notice that there are lines being drawn in the background. A script is transmitting your clicks to the server, and when you load a new page, all of your clicks are drawn into the background image using PHP and GD. It is not meant as a serious tool, at least not until I have added an inferface to control the drawing methods.
I have only created a few trivial examples of what AJAX can do, but I see great things on the horizon, for my own website and the web in general.
Check out
http://maps.google.com for a really cool example of how intelligent browser/server communication can make a common task more intuitive.