title: What's appication ID and how do i use it?
body: <p>In a Gtkmm application, one of the first line is:</p>
  <pre class="lang-cpp prettyprint-override"><code>Glib::RefPtr&lt;Gtk::Application&gt; app = Gtk::Application::create(argc,argv,&quot;org.gtkmm.examples.base&quot;);
  </code></pre>
  <p>I know that <code>org.gtkmm.examples.base</code> is an application ID. Some questions about application IDs:</p>
  <ol>
  <li>What is an application ID?</li>
  <li>What is the purpose of an application ID?</li>
  <li>How do I write it?</li>
  </ol>