Android apps typically consist of some, or all, the four different components listed following:
Activities
Services
Broadcast receivers
Content providers
An activity provides a screen with which users can interact in order to do something. Sometimes, it could include a few fragments inside. A fragment represents a behaviour or a portion of user interface in an activity. Users can perform operations such as making a call, sending an SMS, and so on. A good example of an activity could be the login screen of your Facebook app. The following screenshot shows the activity of the calculator application:
A service can perform long-running operations in the background and does not provide a user interface. If you take the music application, you can close all of its screens after selecting the song of your choice. Still the music will be playing in the background. The following screenshot shows the services running on my device: