Drew McLellan:

"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."

02/10/05 update: by the same author: Very Dynamic Web Interfaces.
03/24/05 update: Dynamic Web Apps and the Uncanny XMLHttpRequest Object!, XMLHttpRequest Usability Guidelines.