Using the Activity Manager, the Android System manages a stack of activities which are in different states (starting, running, paused, stopped, destroyed).
Actually the Activity Manager does more than manage activities. It was named that because in the very early implementations of Android all we had were activities (before we had even implemented multiple processes yet), but in the ultimate implementation the activity manager would probably be better called the "application manager" -- it decides when to start processes, and manages all of the top-level components (activities, receivers, services, content providers) running in them.
