$watch() will be triggered by:
$scope.myArray = [];
$scope.myArray = null;
$scope.myArray = someOtherArray;
$watchCollection() will be triggered by everything above AND:
$scope.myArray.push({}); // add element
$scope.myArray.splice(0, 1); // remove element
$scope.myArray[0] = {}; // assign index to different value
$watch(…, true) will be triggered by EVERYTHING above AND:
$scope.myArray[0].someProperty = "someValue";
JUST ONE MORE THING…
$watch() is the only one that fires when an array is replaced with another with the same exact content. For example:
$scope.myArray = ["Apples", "Bananas", "Orange" ];
var newArray = [];
newArray.push("Apples");
newArray.push("Bananas");
newArray.push("Orange");
$scope.myArray = newArray;
Below is a link to an example JSFiddle that uses all the different watch combinations and outputs log messages to indicate which “watches” were triggered: