Angular provides built-in directives.
Some directives override built-in HTML elements, such
<form> tag which is overrided by
AngularJS added lots of functionality, such as validation.
Other built-in directives have ng- namespace prefix.
Some built-in directives do not have an HTML counterpart, such as the ng-controller directive.
All directives prefixed with the ng namespace are part of the built-in directives.
When building our own directives we should not use ng- as namespace.