C++ FAQ Celebrating Twenty-One Years of the C++ FAQ!!!
(Click here for a personal note from Marshall Cline.)
Section 19:
19.1 Is inheritance important to C++?
19.2 When would I use inheritance?
19.3 How do you express inheritance in C++?
19.4 Is it OK to convert a pointer from a derived class to its base class?
19.5 What's the difference between public, private, and protected?
19.6 Why can't my derived class access private things from my base class?
19.7 How can I protect derived classes from breaking when I change the internal parts of the base class?
19.8 Protected data vs. private data with protected access functions?
19.9 Okay, so exactly how should I decide whether to build a "protected interface"?
[19.2] When would I use inheritance?

As a specification device.

Human beings abstract things on two dimensions: part-of and kind-of. A Ford Taurus is-a-kind-of-a Car, and a Ford Taurus has-a Engine, Tires, etc. The part-of hierarchy has been a part of software since the ADT style became relevant; inheritance adds "the other" major dimension of decomposition.