From Joomla! Documentation

Asynchronous JavaScript and XML or AJAX, is a term for a group of interrelated web development techniques used for creating interactive web applications or commonly known as web 2.0. Ajax allows web applications to retrieve data from the server asynchronously in the background without interfering with the display and behavior of the existing website page. Data is retrieved using the XMLHttpRequest object or through the use of Remote Scripting in browsers that do not support it.

Using AJAX in Joomla

JavaScript Resources:

  1. jQuery
  2. MooTools
  3. Prototype
  4. Script.aculo.us
  5. Help I Don’t Know JavaScript Blog