I seem to gather from the limited info I can find<http://drupal.org/node/22268>that it's used to simple store user data in another, possibly non-drupal database on the same database server... am I right? If so, how do I tell it which table in that database, and which field in that table, maps to user name, etc.? Or, is it used to simply allow multiple drupal instances to use the same user base?
yes, it is a mapping between the drupal userid and an external userid. the decision of what table and what column in the external system is entirely up to the module which 'owns' the authmap record. see the 'module' column of authmap table. that module has logic either hard coded or in settings about connection details to foreign system. hope that makes sense.