[duck]

So they just add content , by posting on the forums??

Yeah,
Someone correct me if I'm wrong but I think a good thing about forums is that the webmaster can enjoy free content being added to his/her site without much work on their part (assuming the webmaster has moderators doing the work for him/her). The search engines pick up the new content and the site gets more traffic. That's the reason everyone whats a forum, right?

Thanks in advance to anyone that wants to respond to this!

[penguinmama]

It would help, I think, to have a specific theme in which the keywords come up often in people's posts. For example, this thread has a lot of "search engines" and "forums" in it, so the search engines will pick that up.

[duck]

Right. And I suppose if you wanted to get really tricksy, you could get on Nichebot and get an idea of some good keywords for your industry. Then you could create the forum around the keyword to be sure the posters use your keywords in their posts.