The basic unit of an Android application is an activity.
An activity by itself does not have a presence on the screen. Instead, it has to draw the screen using Views and ViewGroups.
A view is a widget that has an appearance on screen.
Examples of views are buttons, labels, and textboxes.
A view derives from the base class android.view.View.
One or more views can be grouped together into a ViewGroup.
A ViewGroup, which is itself a special type of view, provides the
layout in which you can order the appearance and sequence of views.
Examples of ViewGroups include LinearLayout and FrameLayout.
A ViewGroup derives from the base class android.view.ViewGroup.
Android supports the following ViewGroups: