An object in a computer program can be loosely defined as something that represents a "thing," like a car or a motorcycle. Objects have states (is the engine running or is it off?), identities (what make is this car?), and behaviors or functions (turn the wheels, accelerate, etc.).

What this means in real terms is that there is a collection of data and functions in the computer program associated with some logical item that represents a thing in the real-world. The type of an object, generally named in some way after what the object represents, is usually called its "class."

A class is a description or template that describes how to build and use an object. An object created from a class description is said to be an "instantiation" or "instantiated object" of that class. A "concrete class is a class from which it is possible...