Index: AbstractFactory Adapter Action Behavioral Bridge Builder Chain of Responsibility Command Composite Creational Cursor Decorator Dependents Facade Factory Method Flyweight Handle/Body Interpreter Iterator Kit Mediator Memento Objects for States Observer Prototype Proxy Publish/Subscribe Singleton State Structural Surrogate Template Token Transaction Virtual Constructor Visitor Wrapper
AbstractFactory aka Kit   Instantiates a class that is determined at runtime
FactoryMethod aka Virtual Constructor   Lets subclasses do object creation: constructor does not help if you do not know the concrete class
Builder   A class that is called to build another class, e.g. document with SAX
Prototype   "Prototypical instance" that is used to make new objects by copying
Singleton   Ensures having one instance per class; get, find, or create returns the same object; constructor is private
Adapter aka Wrapper   Converts one interface to another
Bridge aka Handle/Body   Separates multiiple implementations; cf. abstract factory + adapter
Composite   Represents "part-whole" hierarchy
Decorator aka Wrapper   Adds functionality to an existing class
Facade   Unified interface to a set of interfaces; provides simple inteface to complex subsystems; decouples subsystems from clients; use for layering
Flyweight   Use it for sharing to support large numbers of objects
Proxy aka Surrogate   Use to control access to another object
Chain of Responsibility   Decouples sender from receiver; use when the actual request handler may not be know in advance
Command aka Action, Transaction   Encapsulates request arguments as an object; good for parameterized actions, undo, queue requests, logging, transactions
Interpreter   Intereprets a language
Iterator aka Cursor   Provides sequential access to elements of an aggregate object
Mediator   Redirects messages, like a PBX. Use it when a distributed behavior should be customizable without subclassing
Memento aka Token   A snapshot of a state; good for undo and when a direct interface for object state whould break encapsulation
Observer aka Dependent, Publish/Subscribe   Good when a change in one object requires changes in others, which need to be notified
State aka Objects for States   Encapsulates changing behavior to avoid switch; use when behavior depends on states, e.g. with large, multipart conditionals

This card is based on Design Patterns by Erich Gamma, Richard Helm, Ralph Johnson, John Vlissides.