To understand ajax you need to understand the client / server / request header relationship...
The client is your browser, it interprets data sent to it in the http header.
The whole thing starts when you type a url in your address bar, the browser sends a request to the server at the resolved IP requesting a page, the server responds by sending back the data for the page in the http header and the page is displayed by your browser.
Ajax is very similar, however rather than requesting a page it requests a "handler" file which performs some function and returns data to your browser, this data is then used by javascript to perform an update of some sort to the existing page.
