How do I transfer just the Forum of a Drupal site to another Drupal site as my staging site is lagging behind the live site, So I need to update it with the live site forum content and then publish my staging site as the live site afterwards.
Use Features for all need of these kind. Module page explains -
Features provides a UI and API for taking different site building components from modules with exportables and bundling them together in a single feature module. A feature module is like any other Drupal module except that it declares its components (e.g. views, contexts, CCK fields, etc.) in its .info file so that it can be checked, updated, or reverted programmatically.
So you can make feature of your users setting as well as forum and you can always track the changes at your staging site and if it's overridden then you can make it again and apply the changes at live site by replacing the created feature and enabling it..