Ajax stands for Asynchronous JavaScript And XML. Although strictly speaking Ajax is not itself a technology, it mixes well-known programming techniques in an uncommon way to enable web developers to build Internet applications with much more appealing user interfaces than those to which we have become accustomed.
When using popular desktop applications, we expect the results of our work to be made available immediately, without fuss, and without our having to wait for the whole screen to be redrawn by the program. While using a spreadsheet such as Excel, for instance, we expect the changes we make in one cell to propagate immediately through the neighboring cells while we continue to type, scroll the page, or use the mouse.