title: GWT: AcceptsOneWidget vs Composite vs IsWidget
body: <p>GWT has a lot of similarly-named (and <em>seemingly</em> similarly-purposed) types:</p>
  
  <ul>
  <li><code>Widget</code></li>
  <li><code>AcceptsOneWidget</code></li>
  <li><code>Composite</code></li>
  <li><code>IsWidget</code></li>
  <li><code>SimplePanel</code></li>
  </ul>
  
  <p>When/where is it appropriate to use each of these? What is their relationship to the <code>RootPanel</code>?</p>