Subclassing UIView and UIScrollView - iOS Programming: The Big Nerd Ranch Guide - page 208

Using UIScrollView
When you want to let the user scroll around your view, you typically make your view the
subview of a UIScrollView , as shown in Figure 6.11 .
Figure 6.11 Object diagram
Scroll views are typically used for views that are larger than the screen. A scroll view
draws a rectangular portion of its subview, and moving your finger, or panning , on the
scroll view changes the position of that rectangle on the subview. Thus, you can think of
the scroll view as a viewing port that you move around a virtual world ( Figure 6.12 ). The
size of the scroll view is the size of this viewing port. The size of the area it can view is the
UIScrollView 's contentSize , which is typically the size of the UIScrollView 's
subview.
Figure 6.12 UIScrollView and its content area