This picture illustrates some of the relationships. Suppose we have a variable named aSet that is an instance of class Set. The solid arrows here represent the instance of relationship. but the Set is simply an object, an instance of class Class. The dashed lines here represent the subclass relationship. Thus, the class Set is a subclass of class Object. But what is Object? It is an instance of a class as well.
But what is Class? It is a subclass of Object (dashed lines), and it is itself a class, so is an instance of class Class.
Well, we fudge here a little bit. A class is actually an instance of a metaclass, which is a little more complex.