An application can be registered to receive broadcasts occurred by system events such as BOOT COMPLETE, SMS RECEIVED, BATTERY LOW, etc. An application may register a receiver for the low battery message for example, and change its behavior based on that information. A broadcast receiver is just a gateway to other components and is intended to do a very minimal amount of work. An activity represents a single screen with a user interface. A service is a component that runs in the background to perform long-running operations or to perform work for remote processes.”]
Source: https://gbhackers.com/android-application-penetration-testing-part-5/