It defines a family of algorithms, encapsulate each one, and make them interchangeable. It lets the algorithm vary independently from clients that use it.

Provide a surrogate or placeholder for another object to control access to it.

It defines a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.

It uses sharing to support large numbers of fine-grained objects efficiently.

It provides a unified interface to a set of interfaces in a subsystem. It defines a higher-level interface that makes the subsystem easier to use.

It attaches additional responsibilities to an object dynamically. It provides a flexible alternative to sub classing for extending functionality.

It arranges objects into tree structures to represent part-whole hierarchies and lets clients treat individual objects and collections of objects uniformly.

It avoids coupling the sender of a request to its receiver by giving more than one object a chance to handle the request. It links the receiving objects and pass the request along the list until an object handles it.

It decouples an abstraction from its implementation so that the two can vary independently.

It specifies the kinds of objects to create using a clone-able instance and creates new objects by copying this instance.

It defines an interface for creating an object, but let subclasses decide which class to instantiate. Thus, it lets a class defer instantiation to subclasses.

It separates the construction of a complex object from its representation so that the same construction process can create different representations.

It provides an interface for creating families of related or dependent objects without specifying their concrete classes.

It converts the interface of a class into another interface clients expect. Let’s classes work together that couldn't otherwise because of incompatible interfaces.