What is a Controller?

A Controller is a plain javascript function which helps the data flow in an angular application and is responsible for binding the data to the view. A Controller is defined in an application by using ng-controller directive. It is defined in the following fashion: 

var app = angular.module("myApp", []);
app.controller('myCtrl', function($scope) {});

It is usually referenced after the module is referenced. $scope is passed as an argument to the controller which is helps controller point to the view it has to control. A controller is the key aspect which helps angular to develop applications in MVC architectural pattern.