See also Squeak Intro Smalltalk is an object-oriented programming language because everything that a Smalltalk programmer deals with is an object , from a number to an entire application.
All objects are an instance of a class.
Classes serve as templates because they define the instance variables for all the class variables and methods. Objects have public and private properties. An object's implementation details are private and are hidden from other objects. An object's public properties are its messages that it understands. This make up its interface.