He introduces it as the combination of Javascript and XML through the use of the XMLHTTPRequest object. He notes that, while Ajax is hailed as the greatest thing by some (with advantages like a lower bandwidth usage), there are still others that don't think it's all that useful.

From there, he visually shows the difference between the standard "enter the data and submit" model that's been used for so long, and the Ajax model of instant feedback. He wraps it all up with some example code and the XML that it can fetch from...