What exactly is watchdog? I am aware of the watchdog table in the Drupal database, but I am not sure how to use it effectively.  Is it simply a message function that adds to the table like a stack in the database for debugging?  I have found old references to modules and odds and ends in content, but have not located a broad discussion of different ways it can be used effectively.  Also, how do I get the reports?
http://api.drupal.org/api/drupal/includes%21bootstrap.inc/function/watchdog/6 talks about the watchdog() call, and you access the reports it generates from admin/reports/dblog provided that you've turned on the optional Database Logging core module (dblog).