A forum (or bulletin board) allows free-form discussions between your staff and your visitors. These work best where there is an established community of like-minded individuals with an interest in a particular topic - connected to your business. If such a community doesn’t already exist then starting a forum may be a risk - an empty forum doesn’t reflect well on your business.
But if you can gather a community of users to your forum you will benefit from a constant flow of new content that keeps bringing people back to your web-site.
Preferred applications:
Others to consider:
vBulletin, phpBB, punBB