Comprehensive list of resources related to Themes and Templates, how to customize them, and how to create them
General Theme Articles
Theme Development
Template File Articles
Template Hierarchy
The Loop
http://www.chipbennett.net/themes/template-hierarchy/
Misc.
Running a Development Copy of WordPress
Function Reference - Complete PHP function reference for WordPress (version 2.1, with links to previous versions)
Global Variables- A list of all global variables created by WordPress
Post Types - Creating new types of posts other than the posts that display on the main loop.
Taxonomies - Creating new types of taxonomies other than the built-in ones.
- Quicktags API
- Shortcode API
- Options API - Details about the generic option storage system.
- Rewrite API - Details about the URL rewriting API.
- Settings API - A reference with examples for adding new settings to existing settings screens.
- Dashboard Widgets API - A reference with examples for adding new widgets to the admin dashboard.