Do I understand Ajax correctly?

1) Nope. The XMLHttpRequest object does exactly what its name implies -- it initiates an HTTP request. This request can be in XML, or HTML, or PHP. At the end of the day, the browser doesn't care, because in an AJAX request, it doesn't parse the request -- you have to do it yourself. So it doesn't automatically render the HTML from an AJAX request.

2) I'm not sure about manipulation (the XHR object may be immutable) but possibly. Would you ever need to extend it or manipulate it? Yes, you can change properties of the object and so on. I apologize. I didn't understand you at first :)

3) Yep.

4) That's a great analogy. It's exactly what happens. Another analogy is a 4 lane highway is to asynchronous as a one-way street is to synchronous. If one car breaks down on the 4 lane highway, the rest can keep moving at their normal speed -- but if one breaks down on the one-way road, everything freezes. :)