Jaxer works with a web server to process and serve web content and respond to callback requests. At a very high level:

1. Jaxer reads and executes the HTML and the JavaScript designated to run on the server;
2. It sends the processed page as HTML to the browser, which reads and executes the processed HTML and JavaScript;
3. The browser JavaScript can then call back the JavaScript functions left on the server.

Of course there's a lot more to it than this... for an in-depth look, see the Jaxer architecture page.