Ajax is of the most important technologies for the development of very interactive web application and due to its features it have become very popular these days.
Ajax is a group of interrelated web development methods used on the client-side to generate interactive web applications.

With Ajax, web applications can send information to, and retrieve information from, a server asynchronously (in the background) without interfering with the display and behavior of the existing page.

Information is usually retrieved using the XMLHttpRequest object. Despite the name, the use of XML is not needed, and the requests require not be asynchronous.

The Ajax term has come to correspond to a broad group of web technologies that can be used to implement a web application that communicates with a server in the background, without interfering with the current state of the page.

Usually in all the web applications, the user enters the data into the form and after that clicks on the submit button to submit the request to the server. Server processes the request and returns the view in new page.

This process is inefficient, time consuming, and a little exasperating for you user if the only the small amount of data exchange is required. For example in an user registration form, this can be frustrating thing for the user, as whole page is reloaded only to check the availability of the user name.

the term Ajax Jesse James Garrett explained that the following technologies are incorporated:

● JavaScript to bring these technologies together

● XML for the interchange of data, and XSLT for its manipulation

● HTML or XHTML and CSS for presentation

● The Document Object Model for dynamic display of and interaction with data

● The XMLHttpRequest object for asynchronous communication