var vs this vs $scope in AngularJS (1.4)

Hi I am learning angular and have lost my mind understanding the difference between 'var', 'this' and '$scope'.
Although I read this link but it went above my head.
When I use ng-controller="myController as ctrl" , I only have access to the variables and function set on this.
Whereas defining ng-controller="myController" I only have access to variables and function set on $scope.
Can some one please explain this topic in depth?