Like most things Drupal-related, you have an overwhelmingly broad range of options available to you if you want to tinker with the code that makes your site run. While this holds an incredible amount of appeal to those with a DIY spirit, it's a bit much for most people.
If, however, you are one of the statistical minority inclined to roll your sleeves up and start messing with such things, this section is for you.
Additionally, the Drupal handbook has an excellent overview of the theming system for Drupal 6, at http://drupal.org/theme-guide.
At the outset, I want to make a couple things clear, First, a detailed analysis of Drupal's theming system is beyond the scope of this book. Second, Drupal offers a lot of flexibility for those who want to mess around with the code; however, just because you can doesn't mean you should.
Additionally, developing a theme is not complete without checking to see how the theme displays in different browsers. This...