FWIW, Categories are just special nodes of type cm:category located under /categoryRoot/generalclassifiable/…, just use the node browser to dig in.
Which in turn means, that all your usual knowledge of Alfrescos database schema applies, which is perhaps not what you wanted to hear 🙂
Seriously, in 9 out of 10 cases it is just a bad idea to access Alfresco's database directly, usually there is a well-known service for what you want. Maybe if you tell us what you want to achieve we can provide a better hint :mrgreen:
Cheers
Gyro