Activities
An activity is the entry point for interacting with the user.
Services
A service is a general-purpose entry point for keeping an app running in the background for all kinds of reasons. A service might play music in the background while the user is in a different app, or it might fetch data over the network
Broadcast receivers
enables the system to deliver events to the app. an app can schedule an alarm to post a notification there is no need for the app to remain running until the alarm goes off.
Content providers
A content provider manages a shared set of app data that you can store in the file system, in a SQLite database, on the web, or on any other persistent storage location that your app can access.
