AJAX is a powerful technology where site viewers can have a grate user experience of browsing a web site.

Mainly it can add/remove dynamic content and add special effects in to web page with out refreshing the web page.
You can read more about AJAX here.
To be honest I'm not an AJAX master :(
but still I can manage to add AJAX to your web pages and allow your users a grate experience of browsing your web site.