Ajax

Using JavaScript technology, an HTML page can asynchronously make calls to the server from which it was loaded and fetch content that may be formatted as XML documents, HTML content, plain text, or JavaScript Object Notation (JSON). The JavaScript technology may then use the content to update or modify the Document Object Model (DOM) of the HTML page. The term Asynchronous JavaScript Technology and XML (Ajax) has emerged recently to describe this interaction model.

Ajax is not new. These techniques have been available to developers targeting Internet Explorer on the Windows platform for many years. Until recently, the technology was known as web remoting or remote scripting. Web developers have also used a combination of plug-ins, Java applets, and hidden frames to emulate this interaction model for some time. What has changed recently is the inclusion of support for the XMLHttpRequest object in the JavaScript runtimes of the mainstream browsers.
The real magic is the result of the JavaScript technology's XMLHttpRequest object. Although this object is not specified in the formal JavaScript technology specification, all of today's mainstream browsers support it. The subtle differences with the JavaScript technology and CSS support among current generation browsers such as Mozilla Firefox, Internet Explorer, and Safari are manageable. JavaScript libraries such as Dojo, Prototype, and the Yahoo User Interface Library have emerged to fill in where the browsers are not as manageable and to provide a standardized programming model. Dojo, for example, is addressing accessibility, internationalization, and advanced graphics across browsers -- all of which had been thorns in the side of earlier adopters of Ajax. More updates are sure to occur as the need arises.

What makes Ajax-based clients unique is that the client contains page-specific control logic embedded as JavaScript technology. The page interacts with the JavaScript technology based on events such as the loading of a document, a mouse click, focus changes, or even a timer. Ajax interactions allow for a clear separation of presentation logic from the data. An HTML page can pull in bite-size pieces to be displayed. Ajax will require a different server-side architecture to support this interaction model.
Traditionally, server-side web applications have focused on generating HTML documents for every client event resulting in a call to the server. The clients would then refresh and re-render the complete HTML page for each response. Rich web applications focus on a client fetching an HTML document that acts as a template or container into which to inject content, based on client events using XML data retrieved from a server-side component.