From: Design Patterns: Elements of Reusable Object-Oriented Software, Gamma, Helm, Johnson, and Vlissides, Addison-Wesley, 1994. Page 18-20:

The two most common techniques for reusing functionality in object-oriented systems are class inheritance and object composition. As we've explained, class inheritance lets you define the implementation of one class in terms of another's. Reuse by subclassing is often referred to as white-box reuse. The term "white-box" refers to visibility: With inheritance, the internals of parent classes are often visible to subclasses.

Object composition is an alternative to class inheritance. Here, new functionality is obtained by assembling or composing objects to get more complex functionality. Object composition requires that the objects being composed have well-defined interfaces. This style of reuse is called black-box reuse, because no internal details of objects are visible. Objects appear only as "black boxes".

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That leads us to our second principle of object-oriented design: Favor object composition over class inheritance.

Christian Lemburg