Allows users to tag content in Drupal I have what I thought would be an obvious question but after searching for an answer I can't find anything even close. 

I would like a way in Drupal to let authenticated users "tag" existing site content by selecting from one of 12 available categories. (They won't need to add any of their own categories.) 

The tags they associate with individual nodes should be stored in a separate table with their uid, the nid and the tid they selected. Later this can be used to show tag distributions for nodes, but that's not immediately needed. 

What's the best way to do this? The users will of course not have edit permissions on the nodes, they just need to be able to tag them.
If you have only 12, or some other manageably small number of categories, I would investigate the Flag module.  Just set up flags named to your categories and you're off and running.