John Pratt over on WPHacks.com has published a great guest post explaining the ins and outs of WordPress Theme Template Pages. These pages are what make up a WordPress theme. Although most theme designers end up doing things their own way, it’s a good idea to figure out the basic flow of a theme in case you want to make one yourself or add template pages to your current theme.
Every time a WordPress page is called the WP ‘engine’, if you will, determines (through process of elimination) what kind of page it is. It’s kind of like a “where am I?” function. WordPress says “what page am I…” and in turn tries to call pages in a specific order. If WP doesn’t find the PHP file it needs it just defaults to the “index.php” file and uses it instead.
If you’re an aspiring theme author, definitely give this post a read, print it off actually and use it as a reference.