Edit in JSFiddle

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

app.factory('Service', function() {
  var Service = {
    foo: 'Shared service'
  };
  return Service;
});

app.controller('Ctrl1', function($scope, Service) {
  $scope.foo = Service.foo;
  Service.foo = 'I am from contoller 1';
});

app.controller('Ctrl2', function($scope, Service) {
  $scope.foo = Service.foo;
});

<div ng-app="sampleApp">
    <div ng-controller="Ctrl1">Controller 1: {{foo}}</div>
    <div ng-controller="Ctrl2">Controller 2: {{foo}}</div>
</div>