I want to create forum for my site that already created with drupal. Should I use forum module that already included in drupal or look for another software like phpBB2? I like to separate users of drupal and users of forum. Users for drupal is limited, but for forum is public.
It depends on how much functionality and features you want to add. Drupal's forum module is a little on the lean side feature-wise, so you may want to go with V-Bulletin and use the module that integrates BV with Drupal, I forget what it is called.
i'm using the Drupal forum, and like donnareed said, it's a bit light on functionality.. If you choose the Drupal-only forum it is easy to allow public access (full, partial, moderated, not moderated) .. I think the vB module that Donna refers to is "DrupalvB"