$watch vs $on in angularjs

$On()

The $on is used to listen to each events.

$on also work to push a function into a stack and returns to another function and clean the original functions from that stack.

$watch()

The angularjs create watch internally.
The watch means that angularjs watches the changes in the variable on the $scope object.

The watches are created using the $scope.$watch() method.

The $scope.watch() method creates a watch of some variables. 

When you register a watch you need to pass two functions

1. One is value function
2. and other is listener function

for example

$scope.$watch(function() {},function() {});