The key workflow of android application is given by the Activity class. The activities are the main entities that you will need to get used too. They control the will give you the interaction with the phone services and the UI design.
Android uses several Activity callbacks to inform you about environmental changes; when the current activity is visible, when it is not, when you should save state, etc…
You can see the basics about Activity lifecycle here.
You should consider Activities as the core of Android application building, they will handle user input and perform the desired tasks depending on the situation.