title: What are 'user' and 'system' times measuring in R system.time(exp) output?
body: <p>I am using <code>system.time(expression)</code> to measure execution time for an R function.</p>
  
  <p>The output I get for the call</p>
  
  <pre><code>system.time(myfunction())
  </code></pre>
  
  <p>is:</p>
  
  <pre><code>    user  system elapsed   
    117.36    5.65  127.86
  </code></pre>
  
  <p>What does 'user' and 'system' measure?</p>