A revenue sharing forum shares earnings with its members since without members there wouldn't be a forum. This is usually done with some kind of affiliate program such as Google AdSense. There would be AdSense advert blocks around the forum. If you reply in a topic you will be entered in a rotator of who's AdSense block shows up and you will earn some of the revenue coming from that topic.
It can also be done by paid to post. Paid to posts usually pay every month. They take total earnings and choose a percent of it that gets paid to members. Once they know how much they want to pay to members they divide that number by the number of posts. Then they award each member money or points for each post that they have.
Above are some of the restrictions for participating in revenue sharing.
Different revenue sharing sites has different ratios and restrictions. Some forums show 50% ads from the users while some show 50% ads for charity purposes.