Virtually every template which has an array in it (for example $post, $userinfo, etc.) has keys directly from the database schema. For example $user[usergroupid] is a valid variable to use in some templates because usergroupid is a name of a column in the user table.

So long story short, install phpMyAdmin (http://www.phpmyadmin.net/) and check out the database structure and schema. Once you learn it you can make pretty as well as functional template mods. :)