One type of android application is called an Activity. An activity is an actual application, with a GUI and all. Simply put, a task is what the user experiences as an "application." It's a group of related activities, arranged in a stack. And a stack is a group of objects from all of the classes, all in one "Application", or activity, that the user is interfacing with. If I am wrong, please tell me so I don't make anything harmful xD

Source : Application Fundamentals | Android Developers