why classes are called abstract data types code example

Example 1: what is an abstract data type

An abstract data type is a type with associated operations,
 but whose representation is hidden...
 
 source: wiki.haskell.org

Example 2: define abstract data type

Abstract Data type (ADT) is a type (or class) for objects whose behaviour is
defined by a set of value and a set of operations.

The definition of ADT only mentions what operations are to be performed 
but not how these operations will be implemented.