Difference between directive controller and normal controller

What is the difference between a normal ng-controller and a controller inside a directive.

<div ng-controller="MyCtrl">
    <directive></directive>
</div>

Below i have a directive inside MyCtrl and my directive also has a controller defined.

var app = angular.module('myApp',[]);

app.directive('map', function() {
    return {
        restrict: 'E',
        controller: function(){}
    }
});

function MyCtrl($scope) {}

1. What is the difference between a normal ng-controller and a controller inside a directive.


2. Why does the directive have a controller function?.