AJAX

Asynchronous JavaScript and XML is a web development technique for creating interactive web applications. The goal of AJAX is to increase the web pages interactivity, speed and usability. It tries to achieve this by exchanging small amounts of data behind the scenes thus eliminating the need for the user to reload the entire web page each time the user makes a change.

AJAX relies on a combination of technologies including

• XHTML and CSS for markup and styling information,
• DOM accessed with a client side scripting language such as JavaScript and JScript
• The XMLHttpRequest object to exchange data asynchronously with the web server
• XML is commonly the format used to exchange data back and forth to the server.

AJAX is really an amalgamation of a number of different component technologies. It's advantages include interactivity and portability. The main disadvantages of AJAX are unexpected behavior of the browser's back button and the difficulty of setting bookmarks. Network latency has also been an issue with AJAX. AJAX also requires that the browser has JavaScript enabled and Internet Explorer 6 additionally requires ActiveX.

With AJAX instead of loading a web page at the start of a session the browser loads an AJAX engine - written in Java Script and not visible to the user. This engine is responsible for rendering the user interface as well as communicating with the server. This AJAX engine allows the user's interaction with the application to happen asynchronously - independent of communication with the server. The AJAX engine commonly exchanges data with the server in XML format instead of HTML + CSS.