Object-Oriented Design (OOD) is a way for the designer of a computer program to specify what a computer program will do, using the way that people think and not using the way that computers think. People think in terms of classes and objects. For example, fruit is a class, and there are different kinds of fruits such as bananas and pineapples. If a computer program designer were specifying to a computer programmer how to create a computer program to manage the fruits in a supermarket, then he or she would specify the program in terms of classes and objects which is how people think.
A computer program called a compiler and/or an interpreter would take the program that the programmer typed in and convert it into a form that a computer can run. Computers really only understand things that are encoded as numbers. Object-Oriented-Design (OOD) uses the power and speed of a computer to allow humans to be more productive in making computer programs, by letting human computer programmers think in terms of how humans think.