The Anatomy of a WordPress Database
If you’re new to WordPress and want to familiarize yourself with how the database is structured for a standard 3.0 installation, check out the codex database description.
This is very helpful if you’re planning to write a plugin. Scroll down for a visual overview of the WordPress database and you’ll be able to see how the tables are related to one another. The table overview section gives a description of each of the tables and lists the relevant areas of WordPress user interface.
Scroll further down and you will find an overview of all the multisite global tables that are created when you enable Multisite. This is one page you’ll want to bookmark for future reference.