title: Rails 3.1: Engine vs. Mountable App
body: <p>Can someone help me understand the differences between a Rails Engine and a Mountable app?  In Rails 3.1, you can create either one with the "rails new plugin <strong><em>_</em>__</strong>" command.</p>
  
  <pre><code>rails plugin new forum --full        # Engine
  rails plugin new forum --mountable   # Mountable App
  </code></pre>
  
  <p>When would you want to use one versus the other?  I know you can package an Engine as a gem, for one.  Is that not the case for Mountable Apps?  What other differences are there?</p>