The android activities represent a single screen with a user interface(UI) of an application and it will act as an entry point for users to interact with an app. If you have used C, C++ or Java programming languages then you must have seen that programs in C, C++ start from main() function. Similarly in the android system program starts with the onCreate() method.
The activity life cycle of the android is as shown below:
The following callbacks are specified by the Activity class. It’s not necessary to implement all the callback methods but it is important that you should understand each one.