ASP.NET AJAX extensions, released in Jan. 2006 as a free toolkit, help Web developers build applications with some of the responsiveness and user interface (UI) features normally associated with desktop applications.

AJAX is a common acronym for Asynchronous JavaScript and XML and describes a technique in which JavaScript in a Web page downloads small amounts of XML data in the background so that the entire page doesn't have to be refreshed when anything changes. For example, both Google Maps and Microsoft's Live Local Maps use AJAX technologies to allow users to scroll a map in a Web page by clicking and dragging. New sections of the map are downloaded in the background and the page can display a new section of map without reloading the page.

ASP.NET AJAX has the following major components:

• Extensions to ASP.NET that make it easier to implement the server portion of an AJAX application
• The Microsoft AJAX Library, a library of client-side JavaScript code that supports a variety of browsers (including Internet Explorer and Firefox) and can be used even if the developer is using other Web servers, such as Apache
• The ASP.NET AJAX Control Toolkit, a set of prebuilt UI components, such as a calendar control, that can be used in AJAX applications.

ASP.NET AJAX can be downloaded from ajax.asp.net.

For more information on ASP.NET AJAX, see "ASP.NET to Support AJAX Development" on page 10 of the Feb. 2006 Update.