Android applications are organized as a collection of components. There are four types of components, and applications can be composed of one or more of each type. A dynamic instance of a component corresponds to an application subset that can be executed independently of the others. So, in many ways, an Android application can be thought of as a collection of interacting components. Android application components come in four flavors:                                                                                                                               1)Activities.
2)Services. 
3)Broadcast receivers.
4)Content providers.