title: NodeJS / Express: what is "app.use"?
body: <p>In the <a href="http://expressjs.com/guide.html" rel="noreferrer">docs for the NodeJS <code>express</code> module</a>, the example code has <code>app.use(...)</code>. </p>
  
  <p>What is the <code>use</code> function and where is it defined?</p>