UIView and UIWindow
A UIView knows how to draw itself on a UIWindow, which represents the app's window - the container for everything you see.
The user interface is composed on many UIViews within UIViews - everything you see on the screen, from buttons to images to controls, are all different kinds of UIView. These normally exist within a hierarchy. For example a panel UIView may contains button UIViews, and buttons UIViews may contain image UIViews.
A UIView handles related events. For example, when you touch a button on the screen, the UIView for that button will handle what to do when this happens.
A UIView can be sent the message setNeedsDisplay when its drawable content changes. This redraws the UIView. This is called when, for example, the text in a UILabel, which is a type of UIView, changes.