First link from below gives you a basic idea of how craigs list works...

http://www.google.com/search?sourceid=navclient-ff&ie=UTF-8&rls=GGIC,GGIC:1970--2,GGIC:en&q=how+does+craigslist+make+money

Forums are like any other website when it comes to monetizing... you get traffic, you show relevant ads and you hope visitors click through to the advertiser... assuming access to the forum is free.

Some forums in the search engine marketing world have become such great references that forum owners have been able to charge minimal fees for access. Get enough minimal fees from people and it adds up.

I've got a couple articles up that might help you...
http://marlincreek.com/content/category/12/37/35/

But the basics are...
- Good content / information / participation
- Niche... cover a smaller topic really freaking well so that it is the go-to place for information
- Relevant - Don't go to far on the niche extreme... if only 1000 people care about it, they probably won't pay your site hosting fees. Reason? Either their will be too few people on the forum or too few potential advertisers to pay for eyeballs.
- Dedication - forums require more "input" than a website... constant moderation and keeping spam out... as well as interacting with the people that submit / ask / answer.

My suggestion... start with a basic website dedicated to the subject and try to build a viewership... and then look to expand to a forum.

You might also want to read...
http://www.problogger.net/archives/2006/02/07/problogger-forum-poll-results/



Marlin Creek - Idea Seeds
http://marlincreek.com