AJAX: Asynchronous JavaScript and XML

AJAX allows the web page that is loaded by your browser to retrieve information form the server and display it in your current web page without having to refresh or reload the entire web page.  It saves time for the user and therefor makes the web page feel more responsive.

When Javascript or jQuery make hidden information visiable - that is not AJAX.  The browser has already loaded all of that information - but it just remains hidden until someone clicks on something to make it visible.

In order to be true AJAX, the web page must go and fetch the additional information from the server and display it.  AJAX is a mixture of Javascript, the web browser and the web server. Google Maps is a good example of AJAX.