If an activity has lost focus but is still visible (that is, a
new non-full-sized or transparent activity has focus on top of your
activity), it is paused. A paused activity is completely
alive (it maintains all state and member information and remains
attached to the window manager), but can be killed by the system in
extreme low memory situations.
If an activity is completely obscured by another activity,
it is stopped. It still retains all state and member
information, however, it is no longer visible to the user so its
window is hidden and it will often be killed by the system when
memory is needed elsewhere.