#Drupal AJAX Framework
AJAX is the de facto standard for having the web browser interact with the server without requiring a full page refresh. The term is an acronym for Asynchronous Javascript and XML, although it has come to be widely used for any partial page refresh interaction. Drupal's specific version of it does not rely on XML at all. In earlier versions, the Drupal version of this was called AHAH, but as a community we chose to start using standard terminology when Drupal 7 was released.
The primary use case for AJAX in Drupal is within forms.
TODO: Write the rest of this. For now, see Ajax Framework on the Drupal API site.