What is the difference between ng-app and data-ng-app?

I have begun to learn about AngularJS and am confused about what the differences are between the ng-app and data-ng-app directives.

Most of these answers are simply saying makes template valid HTML, or HTML Validator Compliant, without explaining what THOSE terms mean, either.

I do not know for sure, but I'm guessing that these terms apply to HTML validation programs that scan your code for standards compliance - kind of like lint. They do not recognize ng-app as a valid attribute. They expect non default HTML attributes to be prefaced with

data-attribute_name_here.

So, the creators of AngularJS have created alternate names for their directives that include the data- in front of them so that HTML validator programs will "like" them.

None in terms of the runtime behavior, those are just different styles of naming directives as described here: http://docs.angularjs.org/guide/directive

Directives have camel cased names such as ngBind. The directive can be invoked by translating the camel case name into snake case with these special characters :, -, or _. Optionally the directive can be prefixed with x-, or data- to make it HTML validator compliant. Here is a list of some of the possible directive names: ng:bind, ng-bind, ng_bind, x-ng-bind and data-ng-bind.

As you can see from reading this the data- can be used to make your HTML pass HTML validator tests/