In Pharo everything is an object, and every object responds to messages. So if you evaluate the following code
Object subclass: #MyClass
the object Object
will create for you its subclass called MyClass. If you don't have any particular superclass in your mind it's advised to subclass from Object
but this brings a tautological confusion into the previous example. Let's say that you want to create a class PriorityStack with class Stack as a superclass, then you need to evaluate:
Stack subclass: #PriorityStack