Heather Solomon, the MVP queen of SharePoint style, has put together a fabulous CSS reference for SharePoint 2007. This covers both Microsoft Office SharePoint Server (MOSS) and Windows SharePoint Services (WSS) and displays the name of the style along with an example and where the style is defined in what file. Brilliant.
I've put together a PDF version of the reference guide if you're into that. You can download the 12 page reference guide from here. Heather's original online version can be found here.