“By using JavaScript, a web page can make requests to a web
server and get responses in the background. The user stays on the same
page, and generally has no idea that script running on the page might
be requesting pages (using GET) or sending data (using POST) off to a
server behind the scenes. […]Say that you have a couple of select lists on your page, and in a
drill-down style the user's selection in the first list determines the
options available in the second. […] By utilising XML HTTP to fetch
the options for the second list behind the scenes, you not only make
the experience a little more slick for the user (no page reloads), but
you also reduce the load on the server as it doesn't have to rebuild
that page. […] Another neat trick you can perform using this
technology is using the server to perform any tricky processes that
have until now been left to the client.”