What is AjaxEvent? How is it different from UpdatePanel or WebService? I am mostly interested in performance and what's actually going on during an AjaxEvent. What is the AjaxEvent sending back to the server? Does the page run through its normal life cycle? Do controls get initialized with form data and viewstate? Are events for controls fired (like dropdown selected index changed)? What is sent back to the client?

I can probably answer alot of these questions with Fiddler and testing, but I figured it will be good for the community to know more about this too.