"Ajax" is short for "Asynchronous JavaScript and XML". XHTML and CSS are used for structure and presentation. Pages are manipulated and displayed using the Document Object Model (DOM). The browser's XMLHttpRequest object is used to transfer data between the client and the server. This data uses XML format. Javascript is used to dynamically interact with all of this.
This HTML page has a link to a javascript file (on the same server). The javascript adds an onmouseover event to every link on the page. Other functions show and hide the preview window, and pick up the x and y positions of the mouse. Also, a function controls the visibility of the window: the window is only visible when the mouse is over the link. CSS is used to style the preview window.
At this point, the hover preview kicks in for all of the links on the page, resulting in some strange effects when you hover over links in the left column. I hope to get that straightened out soon. If you want to keep things neat, stick to the four links below: