This document discusses the observer design pattern. It defines the observer pattern as establishing a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically. It provides examples of when to use the observer pattern, such as with a stock market notifying individual stocks of changes. The document outlines the structure of the observer pattern, provides class and sequence diagrams, and includes a code demo of implementing the observer pattern with products and subscribers.