What exactly are expressions that produce *side effects*?
<p>I have trouble understanding this paragraph in the page <a href="https://developer.mozilla.org/en/JavaScript/Reference/Operators/Special/void" rel="nofollow">https://developer.mozilla.org/en/JavaScript/Reference/Operators/Special/void</a>:</p>

<blockquote>
  <p>This operator allows inserting
  expressions that produce side effects
  into places where an expression that
  evaluates to undefined is desired.</p>
</blockquote>

<p>What exactly are expressions that produce <em>side effects</em> ?</p>
