Welcome to the world of Ajax, the technology that lets Web software act like desktop software. One of the biggest problems with traditional Web applications is that they have that “Web” feel — you have to keep clicking buttons to move from page to page, and watch the screen flicker as your browser loads a new Web page.
Ajax is here to take care of that issue, because it enables you grab data from the server without reloading new pages into the browser.
Ajax is Asynchronous Java Script and XML. Here on sending request to the server, one needn’t wait for the response. Other operations on the page can be carried out. Hence, Asynchronous. On the other hand, Java script sends an HTTPRequest to the server and waits for the XML response.
Ajax is a part of Java Script programming. Java Script is used to manage and control a web page once downloaded. Ajax does not need to wait for the whole page to download.
Use of Ajax can reduce connections to the server since the script has to be requested once.
Most Wanted Ajax Techniques: 50+ Examples and Tutorials article.