Web scripts that provide JSON responses are often invoked directly from within a web browser by using the XMLHttpRequest object. This is a technique popularly known as AJAX. For security reasons, solutions like these can run into cross-domain issues, a restriction that requires you to proxy your requests on the server side. Typically, to work around these issues, public services, such as Yahoo! JSON Services, provide a callback mechanism.
Web scripts also provide this mechanism, which wraps the JSON response text in parentheses and a function name of your choosing. A callback is invoked by adding the following URL query parameter to the web script request: alf_callback=<function>
The function parameter specifies the name of a client-side JavaScript function to invoke.