Ajax has moved from an obscure and rarely used technology to the hottest thing since sliced bread. In case you have not heard of Ajax, Curt Hibbs narrates the history of Ajax in 60 seconds or less:
XMLHttpRequest must have been one of the Web's best kept secrets. Since its debut in 1998, few sites have used it at all, and most developers, if they even knew about it, never used it. Google started to change that when it released a series of high-profile web applications with sleek new UIs powered by XMLHttpRequest. The most visually impressive of these is Google Maps, which gives you the illusion of being able to drag around an infinitely sizable map in its little map window.
Ajax on Rails has useful information on how to pimp your web.