Class vs ClassNames

components.my-component.js

import Ember from 'ember';

export default Ember.Component.extend({
});

components.my-other-component.js

import Ember from 'ember';

export default Ember.Component.extend({
  classNames: 'orange'
});

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({

});

styles.app.css

body {
  margin: 12px 16px;
  font-family: 'Helvetica Neue', Helvetica, Arial, sans-serif;
  font-size: 12pt;
}

.yellow {
  padding: 20px;
  background-color: yellow;
}

.orange {
  padding: 20px;
  background-color: orange;
}

.border-green {
  border: 1px solid green;
}

.big-text {
  font-style: bold;
  font-size: 2em;
}

.give-me-space {
  margin: 20px;
}

templates.application.hbs

<h1>Class vs ClassNames</h1>

{{#my-component as |comp|}}
  Parent
  {{#comp.child class="give-me-space"}}Child{{/comp.child}}
  {{#comp.child classNames="give-me-space"}}Child{{/comp.child}}
{{/my-component}}

templates.components.my-component.hbs

{{yield (hash child=(component "my-other-component" class="big-text" classNames="border-green"))}}

templates.components.my-other-component.hbs

{{yield}}

twiddle.json

{
  "version": "0.11.0",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "2.10.2",
    "ember-data": "2.11.0",
    "ember-template-compiler": "2.10.2",
    "ember-testing": "2.10.2"
  },
  "addons": {}
}