What is AJAX?
AJAX is a culmination of existing technologies that allows web developers to provide a new level of interactivity to their applications.
Because AJAX "changes" the way we expect a browser to operate, potential applications, methodologies, best practices, and caveats are still being discovered. Though the technologies AJAX build on mature and stable, some older browsers do not have access to the XMLHTTP object and cannot execute web applications utilizing AJAX. Thus, it is in the developer's hands to decide when and where best to implement AJAX and provide a fallback method for communicating with the server.