Activity Cycle : 

The State of the Android process is determined by status of the activities and components that make up the applications that it hosts. The current state of the activity is determined by Activity Stack.

 

As shown in the diagram, new activities are pushed on the top of the stack when they are started. The current activity is at the top of the stack until it is pushed down the stack by the new activity , or popped of the stack when it exists or the user navigates to the previous activity.