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
One or more views can be grouped together into 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
ViewGroup derives from the base class
Android supports the following ViewGroups: