A scroll view (UIScrollView) is a view whose content is larger than its bounds. To reveal a desired area, the user can scroll the content by dragging or flicking, or you can reposition the content in code.
Think of the scroll view as consisting of two things:
Although it is useful to think of the scroll view in this way, the truth is far simpler. The scroll view isn’t really specially window-like; it’s just a view (whose
clipsToBounds is usually YES). And the content isn’t really a separate thing; it’s just a set of parameters for positioning the scroll view’s subviews. When the scroll view scrolls, what’s really changing is the scroll view’s own bounds origin; the subviews are positioned with respect to the bounds origin, so they move with it. (See Chapter 14.)
However, a scroll view does bring to the table some nontrivial additional abilities: