The AppCompatActivity class is a subclass of Activity. It lives in the AppCompat Support Library, and it’s desigend to work with the AppCompat themes. Your activity needs to extend the AppCompatActivity class instead of the Activity class whenever you want an app bar that provides backward compatibility with older versions of Android.
As AppCompatActivity is a subclass of the Activity class, everything we know about activities still applies.
AppCompatAActivity works with layouts in just the same way, and inherits all the lifecycle methods from the Activity class. The main difference iss that, compared to Activity, AppCompatActivity contains extra smarts that allow it to work with the themes from the AppCompat Support Library.