An Android application can consist of a multitude of different components:
Activities: These are user-facing components that present a UI with which to interact.
Services: These are processes that work in the background and don't have a visible UI. For example, a service might be responsible for polling a mail server for new e-mails.
Content providers: These components make parts of your application data available to other applications.
Intents: These are messages created by the system or applications themselves. They are then passed on to any interested party. Intents might notify us of system events such as the SD card being removed or the USB cable being connected. Intents are also used by ...