OOP design principles

Here is a page I would like to keep a reference to. The Principles of OOD

What is object oriented design? What is it all about? What are it's benefits? What are it's costs? It may seem silly to ask these questions in a day and age when virtually every software developer is using an object oriented language of some kind. Yet the question is important because, it seems to me, that most of us use those languages without knowing why, and without knowing how to get the the most benefit out of them.