The forum of Drupal is basically similar to a page with comments. Natively its a threaded discussion and it requires a lot of customization to make it look similar to phpBB (which isn't really that easy). Also it lacks many features of a full fletched BB but many can be installed as a drupal module. Let me just say its a lot of work to make a Drupal forum look good and have basic things like moving posts and merging ... The forum on Drupal.org is pretty much what you get out of the box http://drupal.org/forum. The rest requires lots of CSS tweaking.