Difference between two function placements

Question

What would be the difference between the following two ways of attaching a method(?) to a parent function:

var Person = function(first, last) {
    this.first = first;
    this.last = last;
};
Person.prototype.personMethod = function() {
    // ...
};

And:

var Person = function(first, last) {
    this.first = first;
    this.last = last;
    this.personMethod = function() {
        // ...
    };
}