Just like other programming languages such as C/C++, the Android system also initiates with a main activity. To do this, we use a callback method onCreate() and after that, a series of callback methods are called to start an activity. 

You can also refer to the image shown below from: https:www.android.com 

The activity class is used to define various events. Let’s understand these methods below:

• onCreate(): This is the first callback method called on the creation of an activity.
• onStart(): This method is called when the activity starts and becomes visible to the user.
• onRestart(): As the name suggests, the method is called when the activity restarts after stopping it. 
• onResume(): This callback method is called when the user starts performing an activity with the application. 
• onPause(): This method is used to pause the activity when the user is no more interacting with the application. 
• onStop(): The method is called when the activity is stopped and the application is no longer visible to the user. 
• onDestroy(): This callback method is initiated when the activity is destroyed by the system due to some error or by the user.