Singleton - is a regular class that is instantiated (created) only ONCE for the duration of the application. It can be used anywhere by any class at any time, much like global variables
CCDirector - singleton class that stores global configuration settings & manages cocos2d scenes
CCNode - defines a lot of common properties & methods. CCNodes are like views with addChild, getChildByTag etc.
CCAction - used to move, rotate, scale nodes or sprites.
@selector(anyMethod:) - Is a way of specifying a specific method (anyMethod).
CCLayer - is typically used to group nodes together & receive touch & accelerometer input if enabled.
CCScene - is always the first node in a scene graph.
CCTransitionScene - allows for many very cool transitions between scenes (page 52).
keeping a weak reference - Not retaining an object whose memory is managed by another class or object
