Ajax is a combination of Web development technologies used for creating dynamic websites. While the term "Ajax" is not written in all caps like most tech acronyms, the letters stand for "Asynchronous JavaScript And XML." Therefore, websites that use Ajax combine JavaScript and XML to display dynamic content.

The "asynchronous" part of Ajax refers to the way requests are made to the Web server. When a script sends a request to the Web server, it may receive data, which can then be displayed on the Web page. Since these events happen at slightly different times, they are considered to be asynchronous. Most Ajax implementations use the XMLHttpRequest API, which includes a list of server requests that can be called within JavaScript code. The data is usually sent back to the browser in an XML format, since it is easy to parse. However, it is possible for the server to send data as unformatted plain text as well.

What makes Ajax so powerful is that scripts can run on the client side, rather than on the server. This means a JavaScript function can make a request to a server after a webpage has already finished loading. The data received from the server can then be displayed on the page without reloading the other content. If a server-side scripting language like PHP or ASP was used, the entire page would need to be reloaded in order for the new content to be displayed.

While you may not realize it, you have probably seen Ajax at work on several different websites. For example, search engines that provide a list of search suggestions as you type are most likely using Ajax to display the suggestions. Image searches that produce more thumbnails as you scroll through the results typically use Ajax to retrieve the continual list of images. When you click "Older Posts" at the bottom of a Facebook page, Ajax is used to display additional postings.

Ajax has helped make the Web more dynamic by enabling webpages to retrieve and load new content without needing to reload the rest of the page. By using Ajax, Web developers can create interactive websites that use resources efficiently and provide visitors with a responsive interface.

Updated April 13, 2011 by Per C.

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