title: What do 'real', 'user' and 'sys' mean in the output of time(1)?
body: <pre><code>$ time foo
  real        0m0.003s
  user        0m0.000s
  sys         0m0.004s
  $
  </code></pre>
  <p>What do <code>real</code>, <code>user</code> and <code>sys</code> mean in the output of time? Which one is meaningful when benchmarking my app?</p>