In the past when a site owner wanted to implement a forum for their website they looked toward specific packages like phpBB or vBulletin. These and other dedicated forum software are powerful software programs that have been around for many years with thousands of active users. For most basic sites these work quite well. The problem comes when you are trying to integrate one of these forum packages into an existing site or if you need to customize the forum.
All the major forum software programs have to be hacked to changed the functionality or alter the look of the site. Custom hacking does pose a problem when it comes to upgrading the forum to a newer. New versions of software packages usually contain bug and security fixes. With each release you would have to manually reapply all the customizations, conduct extensive testing, and hope that all goes well.
Another problem with these forum programs is that in general they are designed for a site that is just a forum. If you want the forum to sit on top of another content management system (CMS) that shares users and has other features that would be shared in different areas of the site you would have to once again hack the code to get this integration. You would also have to either hack the look of your CMS or hack the forum software so you would get a seamless integration between the two.
Drupal has a basic forum module in it. With some additional modules enabled a Drupal forum can replicate the functions and look and feel of these other dedicated forum solutions. The very nature of Drupal separates customizations or hacks from the core code. Any functionality of Drupal can be overridden by programming custom hooks. The look and feel of the forums in Drupal can also be overridden at the theme layer to create the exact look desired. Once again these customization are separated from the core code. This separation allows the core Drupal code and modules to be easily updated as new security and other updates are released.
A great case study has been published on janaksingh.com. The study goes into details how a large forum was converted from phpBB to Drupal. In the end they were able to get the same functionality and look on Drupal than they had when using phpBB with all the advantages that Drupal offers.