Microsoft has finally released AJAX 1.0.
AJAX stands for 'Asynchronous Java And Xml'.
One of the most important things I like about AJAX is the fact that it allows you to refresh a part of your website without having to do a complete postback. This means, for example, that you can have a photo gallery, slideshow through a collection of photos on your hard disk without refreshing the entire site.
Check the website, http://ajax.asp.net/default.aspx and find out more about it.