Your application's action controllers contain your application workflow, and do the work of mapping your requests to the appropriate models and views.

An action controller should have one or more methods ending in "Action"; these methods may then be requested via the web. By default, Zend Framework URLs follow the schema /controller/action, where "controller" maps to the action controller name (minus the "Controller" suffix) and "action" maps to an action method (minus the "Action" suffix).

Typically, you always need an IndexController, which is a fallback controller and which also serves the home page of the site, and an ErrorController, which is used to indicate things such as HTTP 404 errors (controller or action not found) and HTTP 500 errors (application errors).

The default IndexController is as follows:

// application/controllers/IndexController.php

class IndexController extends Zend_Controller_Action
{

    public function init()
    {
        /* Initialize action controller here */
    }

    public function indexAction()
    {
        // action body
    }
}

And the default ErrorController is as follows:

// application/controllers/ErrorController.php

class ErrorController extends Zend_Controller_Action
{

    public function errorAction()
    {
        $errors = $this->_getParam('error_handler');

        switch ($errors->type) {
            case Zend_Controller_Plugin_ErrorHandler::EXCEPTION_NO_ROUTE:
            case Zend_Controller_Plugin_ErrorHandler::EXCEPTION_NO_CONTROLLER:
            case Zend_Controller_Plugin_ErrorHandler::EXCEPTION_NO_ACTION:

                // 404 error -- controller or action not found
                $this->getResponse()->setHttpResponseCode(404);
                $this->view->message = 'Page not found';
                break;
            default:
                // application error
                $this->getResponse()->setHttpResponseCode(500);
                $this->view->message = 'Application error';
                break;
        }

        $this->view->exception = $errors->exception;
        $this->view->request   = $errors->request;
    }
}

You'll note that (1) the IndexController contains no real code, and (2) the ErrorController makes reference to a "view" property. That leads nicely into our next subject.