title: Difference between Real User ID, Effective User ID and Saved User ID
body: <p>I am already aware of the <em>real user id</em>. It is the unique number for a user in the system.</p>
  <p>On my system, my <code>uid</code> is</p>
  <pre><code>$ echo $UID
  1014
  $
  </code></pre>
  <p>What do the other two IDs stands for?</p>
  <p>And what is the use of <em>effective user id</em> and <em>saved user id</em> and where do we use them in the system?</p>