title: What is "metrics" in Keras?
body: <p>It is not yet clear for me what <code>metrics</code> are (as given in the code below). What exactly are they evaluating? Why do we need to define them in the <code>model</code>? Why we can have multiple metrics in one model? And more importantly what is the mechanics behind all this? 
  Any scientific reference is also appreciated.</p>
  
  
  
  <pre class="lang-python prettyprint-override"><code>model.compile(loss='mean_squared_error',
                optimizer='sgd',
                metrics=['mae', 'acc'])
  </code></pre>