AJAX and asp.net

I've written a little code that shows how to AJAX-ify an aspx web form.

1. Write a javascript function that creates an xmlHttpRequest object
2. Write a javascript function that calls the function in #1 and passes it a link to post to the code behind
3. Write your code behind to respond to an event connected with #2 and return a string
4. Remember to assign the value of the returned string to a div or something in the HTML

I'll post the code once I get my laptop on my home network so I can copy paste it. What is important here is to understand why AJAX, while old and nothing new, is a key to making web apps more equal to client/server/desktop applications.