Template:Simple Application Activity Lifecycle
One of the application component type is called Activity and provides a screen for the user to interact with.
The activity object could be created from a subclass of the Activity class or one of its subclasses.
Based of the user input on the activity screen, the operating system calls into your class some important methods. The figure from this source illustrates the life cycle of an android activity component.
Here is a sample that illustrates the activity life cycle.
The steps in building this app and the detailed lifecycle states are defined in the how to build a simple Android app tutorial