What is Ajax?

Ajax is a way of developing Web applications that combines:
  - XHTML and CSS standards based presentation
  - Interaction with the page through the DOM
  - Data interchange with XML and XSLT
  - Asynchronous data retrieval with XMLHttpRequest
  - JavaScript to tie it all together

원문 : http://webdesign.about.com/od/ajax/a/aa101705.htm