AngularJS Example:

HTML

<link rel="stylesheet" href="http://twitter.github.com/bootstrap/assets/css/bootstrap.css">
<div ng-app>
  <h2>Todo</h2>
  <div ng-controller="TodoCtrl">
    
     <span class="ng-class:myClass">Demo text</span>
     <span ng-class="myClass">Demo text</span>
  </div>
</div>

CSS

.red {
  
  color: red;
}

JavaScript

function TodoCtrl($scope) {
  $scope.myClass = "red";
}