Creating Themes — Wordpress vs Drupal

There's some differences in how the two systems work with theming, though they are actually pretty similar. Both use some light PHP in the main template files, and (though some themes on both sides don't always follow the rule) they keep more complex PHP functions outside in another file (in Drupal, it's called template.php). Drupal has a main template for your layout called page.tpl.php, and many optional sub-templates that render things within that. Most themes come with a number of the most common ones, though there are quite a few more possible when needed (and with some simple code snippets you can even add templates for just about anything). You can learn more at Core templates and suggestions and Working with template suggestions. Once you get the hang of theming in Drupal I think you will find it quite a bit more flexible than any other system (at least that's the case for me). The "Themer tool" from Devel module is also very handy for telling you what template file is affecting anything on the page you click on.

Here's a guide that shows how the main/basic components of a Drupal theme fit into any other pre-existing theme: http://drupal.org/node/313510

Hope this helps.