Introduction to Abject-Oriented Programming.

Sample:

Inheritance
Inheritance is a way to retain features of old code in newer code. The programmer derives from an existing function or block of code by making a copy of the code, then making changes to the copy. The derived code is often specialized by adding features not implemented in the original. In this way the old code is retained but the new code inherits from it.

Programs that use inheritance are characterized by similar blocks of code with small differences appearing throughout the source. Another sign of inheritance is static members: variables and code that are not directly referenced or used, but serve to maintain a link to the original base or parent code.