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Asynchronous JavaScript And XML (AJAX) allows to make a call to a http server, load external content into the existing page without having to refresh the entire page. This can savs bandwidth because loading the header and footer all over again is not necessary. A disadvantage of AJAX is that it breaks the history of the browser. You can not use the "back" button to reverse to a previous state. Using AJAX, one can write full applications like Ajax write, which looks like MS Word but runs from your browser. The webpage you just read is one of the simplest demonstrations, one can find:

Click into this field to load the content of an external file into this box.


Simplicity, Clarity, Generality B.W. Kernighan, R. Pike, in "The Practice of Programming".