Let’s say we want to organize a list of books and we want to put those books under a particular genre. For instance:
Web Programming: Head First HTML, PHP In A Nutshell, Ajax Design Patterns
Philosophy: Arcades, Negative Dialectics, Phenomenology of the Spirit
Art History & Theory: Art Since 1900, First Person
I might make a top level list that contains the categories and then have a sub list for each category that contains appropriate titles.


<h1>My Books</h1>
<ul>
  <li><h2>Web Programming</h2>
    <ul>
      <li>Head First HTML</li>
      <li>PHP In A Nutshell</li>
      <li>Ajax Design Patterns</li>
    </ul>
  </li>
  <li><h2>Philosophy</h2>
    <ul>
      <li>Arcades</li>
      <li>Negative Dialectics</li>
      <li>Phenomenology of the Spirit</li>
    </ul>
  </li>
  <li><h2>Art History &amp; Theory</h2>
    <ul>
      <li>Art Since 1900</li>
      <li>First Person</li>
    </ul>
  </li>
</ul>

The former code would display something like this:

My Books

  • Web Programming

    • Head First HTML
    • PHP In A Nutshell
    • Ajax Design Patterns

  • Philosophy

    • Arcades
    • Negative Dialectics
    • Phenomenology of the Spirit

  • Art History & Theory

    • Art Since 1900
    • First Person