Behaviour of named and anonymous function

Question

Check the following cases:-

function x() {
    console.log('x');
};
x(); //outputs x

var y = function () {
    console.log('y');
};
y(); //outputs y

var z = function w() {
    console.log('zw')
};
z(); //outputs zw;
w(); //outputs .. w is not defined.

Can anyone explain me this behaviour..?