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from: :OReilly.JavaScript.The.Definitive.Guide.5th.Edition.Aug.2006

 It turns out that every JavaScript object includes an internal reference to another object, known as its prototype object. Any properties of the prototype appear to be properties of an object for which it is the prototype. Another way of saying this is that a JavaScript object inherits properties from its prototype.

In the previous section, I showed that the new operator creates a new, empty object and then invokes a constructor function as a method of that object. This is not the complete story, however. After creating the empty object, new sets the prototype of that object. The prototype of an object is the value of the prototype property of its constructor function. All functions have a prototype property that is automatically created and initialized when the function is defined. The initial value of the prototype property is an object with a single property. This property is named constructor and refers back to the constructor function with which the prototype is associated. (You may recall the constructor property from Chapter 7 ; this is why every object has a constructor property.) Any properties you add to this prototype object will appear to be properties of objects initialized by the constructor.