Inheritance

... is where once class derives from another, thereby aquiring the interface and implementation of the derived class (based on scoping rules of course). This allows for a significant amount of code reuse and for adapting existing, proven, tested code only as needed to create the needed functionality.

In a more sing-song way, inheritance represents and 'is-a' relationship between one class and another. For example, a Dog 'is-a' Mammal and would inherit several features from that class such as live birth, suckling young, warm-blooded and so on.