Ajax: A New Approach to Web Applications

http://www.adaptivepath.com/publications/essays/archives/000385.php

Every user action that normally would generate an HTTP request takes the
form of a JavaScript call to the Ajax engine instead. Any response to a
user action that doesn’t require a trip back to the server ? such as
simple data validation, editing data in memory, and even some navigation
? the engine handles on its own. If the engine needs something from the
server in order to respond ? if it’s submitting data for processing,
loading additional interface code, or retrieving new data ? the engine
makes those requests asynchronously, usually using XML, without stalling
a user’s interaction with the application.