After creating an android project, the first thing you will notice is the activity class. An activity is the primary class for user interaction, it is a single-focused task that a user can do.
Each activity should be modular and support one-focused thing a user can do in an app. For example, one that handles only the login of your application. It handles and takes care of the UI you will need for that specific window, using setContentView(view)
Activity Life Cycle
In simple words, an activity can be divided in three states: