There is a cronic problem in object-oriented programming; often, it seems, in all of computer science. This is that we use different terms for the same concept, and the same terms to mean different concepts. Thus, the idea of a class is, in Apples Object Pascal called an object type, and in Objective-C termed a factory object. We have already been using the terms instances and objects interchangably. Message passing is sometimes termed method lookup, or in C++ termed member function invocation, or sometimes even method binding. What in most languages are termed methods are, in C++, called member functions, or sometimes method functions. And what we will term inheritance is sometimes called subclassing (we will occasionally use that term ourselves), and sometimes called extension.
The book uses a simple and consistent vocabulary, independently of the language being discussed. Where there is doubt concerning the meaning of a term you can consult the glossary at the back of the book.