title: Difference between a class and its instance
body: <p>How does Ruby internally differentiate between a class and its instance like <code>MyClass</code> and <code>obj</code> below? What does it do to allow creation of instances of <code>MyClass</code> but not of <code>obj</code>?</p>
  
  <pre><code>MyClass = Class.new
  obj = MyClass.new
  </code></pre>