The HTML attribute required="required" is a statement telling the browser that this field is required in order for the form to be valid. (required="required" is the XHTML form, just using required is equivalent)
The Angular attribute ng-required="yourCondition" means 'isRequired(yourCondition)' and sets the HTML attribute dynamically for you depending on your condition.
Also note that the HTML version is confusing, it is not possible to write something conditional like required="true" or required="false", only the presence of the attribute matters (present means true) ! This is where Angular helps you out with ng-required.
I would like to make a addon for tiago's answer:
Suppose you're hiding element using ng-show and adding a required attribute on the same:
<div ng-show="false">
<input required name="something" ng-model="name"/>
</div>
will throw an error something like :
An invalid form control with name='' is not focusable
This is because you just cannot impose required validation on hidden elements. Using ng-required makes it easier to conditionally apply required validation which is just awesome!!
Source: Stackoverflow.com