AJAX is a technique for dynamically adding new data to a web page that's already loaded. It technically stands for asynchronous JavaScript and XML, but the original meaning has been diluted such that AJAX also refers to synchronous JavaScript and JSON, as well. Think of it as DHTML that can dynamically retrieve data from the server in the background.
As JavaScript has matured greatly over the last few years and browser support is generally solid, so there's no reason to shy away from its use. A little JavaScript can greatly improve the user experience for users that choose to enable it.
In this section, we'll improve the AddressBook user interface a bit by adding some AJAX. The end goal is to be able to edit addresses "in place". Instead of clicking "edit" and waiting for the address editing page to load, the text fields will turn into textboxes and the user can begin editing immediately. When he's done, clicking Submit will submit the address for validation and the process will proceed...