Are you responsible for a Drupal site that somebody else built? My October 2016 presentation at DrupalCamp Atlanta will help you understand your site.
This presentation will step you through incrementally understanding your Drupal site. It covers some Drupal basics and resources for learning more. It also looks at Drupal techniques to accomplish common CMS tasks, and shows clues to identify the techniques used in your inherited site. Finally, it provides some examples of bad practices you might find in your inherited site, and how to get past them.