vBulletin is a powerful community solution that makes it easy to add a forum messaging system to your web site. vBulletin is developed using PHP and MySQL (a highly efficient open-source database). This solid back end technology allowed us to develop a product that is both amazingly fast, yet reassuringly stable.

Key features include:
* Fast and efficient database back end
* Template driven interface
* Powerful search engine
* Language system
* Custom user fields
* Extensive admin control panel
* Unlimited forums/posts
* Inline moderation
* Flexible plugin system

More - http://www.vbulletin.com/features.php