How does Drupal know which Entity View Mode to use? When Entity View Modes are defined, how does Drupal know which to use? Are they only useful with Views, or can they be used in the node display as well? If so, is that specified in the template, or somewhere else?
With a tiny bit of glue code you can use specific node template files for different view modes, so they can become pretty useful:

function MYMODULE_preprocess_node(&amp;$vars) {
  if($vars['view_mode'] == 'some_type') {
    $vars['theme_hook_suggestions'][] = 'node__' . $vars['node']-&gt;type . '__some_type';
  }
} 


That will allow you to use template files with names like node--article--some-type.tpl.php specifically targeted to that view mode.

Other than that MPD's answer has all of the other common cases covered.