This method is most often used to get a reference to the global application object, in which case it can be called with no arguments. In this example, the application object is obtained so that a test can be made to see if the code is running on the front-end (client is 'site') or the back-end (client is 'administrator').
$app =& JFactory::getApplication();
if ($app->isSite()) echo 'Client is site';
if ($app->isAdmin()) echo 'Client is administrator';