what is Yii::app() in php framework yii

Yii is the Yii singleton: http://www.yiiframework.com/doc/api/1.1/Yii

Yii::app() returns the instance of CApplication: http://www.yiiframework.com/doc/api/1.1/CApplication

Which has many handy methods and variables such as user (referenced by Yii::app()->user which was information about the user)