Here are two cheatsheets showing the table-structure of WordPress versions 2.3 and 2.2.2. Click the thumbnails to download the PDF.

WordPress 2.3

WordPress 2.2.2

I created these to help transition some custom category queries from WordPress 2.2.2 over to the terms tables in WordPress 2.3. Table keys are in bold.The pages were generated by a small AppleScript Studio app I never quite cleaned up enough to release, it reads MySQL dumpfiles then spits out nice looking tables in OmniGraffle.