Ajax is an approach or pattern to web development that uses client-side scripting to exchange data with a web server. This approach enables pages to be updated dynamically without causing a full page refresh to occur (the dream, we presume, of every web developer). As a result, the interaction between the user and the application is uninterrupted and remains continuous and fluid. Some consider this approach to be a technology rather than a pattern. Instead, it’s a combination of related technologies used together in a creative way.
Some credit remote scripting as the precursor to Ajax development. Prior to the XMLHttpRequest object, remote scripting allowed scripts running in a browser to exchange information with a server. For more about remote scripting, read http://en.wikipedia.org/wiki/Remote_Scripting.
the Ajax programming pattern consists of a set of existing technologies brought together in an imaginative way, resulting in a richer and more engaging user experience. The following are the main pillars of the Ajax programming pattern and the role they play in its model:
Document Object Model (DOM) —Defines the structure of a web page as a set of
the DOM is leveraged to effectively redraw portions of the page.
Cascading Style Sheets (CSS)—Provides a way to define the visual appearance
of elements on a web page. CSS is used in Ajax applications to modify the exterior of the user interface interactively.
XMLHttpRequest—Allows a client-side script to perform an HTTP request. Ajax applications use the XMLHttpRequest object to perform asynchronous requests to the server as opposed to performing a full-page refresh or postback.