Types vs. Classes

A type is an abstract interface. Types generally represent nouns, such as a person, place or thing, or something nominalized.

A class is a concrete data structure and collection of subroutines. It represents an implementation of the type.

An object's class defines how the object is implemented .The class defines object's internal state and the implementation of its operations.

In contrast, an object's type only refers to its interface - a set of requests to which it can respond.

An object can have many types, and objects of different classes can have the same type.

Example

For example, one might implement the type Stack with two classes: SmallStack (fast for small stacks, but scales poorly) and ScalableStack (scales well but high overhead for small stacks).