title: What does ContentPlaceHolderID mean?
body: <p>In a LogOn view, I have the following code:</p>
  
  <pre><code>&lt;asp:Content ID="Content2" ContentPlaceHolderID="MainContent" runat="server"&gt;
  </code></pre>
  
  <p>What does this fragment mean and where is "MainContent" defined?</p>
  
  <p><strong>EDIT</strong></p>
  
  <p>Thanks for your replies. I now understand how it works, but where is "MainContent" defined? Is it defined in the master page?</p>