Objective-C - iOS Programming: The Big Nerd Ranch Guide - page 56

Subclassing an Objective-C Class
Classes, like NSMutableArray , exist in a hierarchy, and every class has exactly one su-
perclass - except for the root class of the entire hierarchy: NSObject ( Figure 2.8 ). A class
inherits the behavior of its superclass, which means, at a minimum, every class inherits the
methods and instance variables defined in NSObject .
As the top superclass, NSObject 's role is to implement the basic behavior of every object
in Cocoa Touch. Three of the methods NSObject implements are alloc , init , and
description . (We sometimes say “ description is a method on NSObject ” and
mean the same thing.)
Figure 2.8 Class hierarchy