OOP concepts prototype keyword use in jquery?

Question

I am studying the oops concepts in jquery But I am facing some problem . First I learn how to create class .

function Car(speed) {
    alert("Class CAR Instantiated");
    this.speed = speed;
}

Car is class speed is parameter , Then I create object of that class like that

var car1 = new Car(40); 

But The problem is that there is keyword prototype ,I think it is used to create function in class ?

Car.prototype.speed= 'Car Speed';
Car.prototype.setSpeed = function(speed) {
    this.speed = speed;
    alert("Car speed changed");
}

But when I call setSpeed function it is called .But when I called speed function (mean alert Car Speed). It is not called

Here is my fiddle ..

http://jsfiddle.net/naveennsit/fJGrA/