"AJAX is not a new programming language, but a technique for creating better, faster, and more interactive web applications. With AJAX, your JavaScript can communicate directly with the server, using the JavaScript XMLHttpRequest object. With this object, your JavaScript can trade data with a web server, without reloading the page. AJAX uses asynchronous data transfer (HTTP requests) between the browser and the web server, allowing web pages to request small bits of information from the server instead of whole pages. The AJAX technique makes Internet applications smaller, faster and more user-friendly."

W3Schools - AJAX = Asynchronous JavaScript and XML

"Any Ajax framework is able to communicate with the server, and thus, to read data or to send it data or commands. In the last case a server-side script is required. The frameworks often add components that make use of the asynchronous communication with the server. The classical examples are buttons, tabbed panels, grids, listboxs and other such widgets. A more innovative example, the 'boxes', are more and more often implemented, and Lightbox and Slimbox are two of them. There are image galleries that place them side by side on the screen and that are making use of Ajax to display them instantanously. Framework may be server-driven also, and in this case, component are created on the server with a scripting language such as PHP, and sent to the browser. Ajax is used to transmit user actions to the server part, and to handle the results. The ability to work offline as it is offered by Google Gears, may be added to the Ajax framework as well."

Ajax Frameworks: Main Tools for Ajax Programming