Android Applications: 

An application in Android is defined by the contents of its manifest.

Each Android application declares all its activities, entry points, communication layers, permissions, and intents through AndroidManifest.xml. Four basic building blocks, when combined, comprise a rich Android application: 

ƒ Activity: The most basic building block of an Android application.

ƒ Intent receiver: A reactive object launched to handle a specific task.

ƒ Service: A background process with no user interface.

ƒ Content provider: A basic superclass framework for handling and storing data.