On NetTuts.com there's a new post for those new to WordPress (or just wanting to figure out more about the internals of the tool) showing how some of the data is structured and how to work with it.
Most WordPress users never come into direct contact with the database and may not even be aware that it's constantly working to populate their site. When WordPress serves up any kind of page, be that the home page, a single post or page or an archive, it's accessing the database to bring up content that editors and administrators have added to the site. In this series of tutorials I'll look in detail at different aspects of the WordPress database.
This post is the first in the series and provides an overview of the database and what kinds of information each one contains. They talk about content types and provide the table structure and relations in a handy graphical form (an ERD). They then go through each of the tables and describe what the data is including link tables, joining the content in different places.