What is AJAXy?
AJAX = Asynchronous JavaScript and XML
That means that a web browser can interact with a webserver without having to do a postback or reload a page. This gives the webpage the ability to act more like a native computer program and provides some seam-less behavior that previously required the browser to redraw the whole page.
Google Maps is a good example of this.. as your mouse or map reaches the boundry of what is visible, instead of the page reloading from the server to get the images for the bordering space, the page can communicate with the server and get the info seamlessly.