There is a growing amount of forums on different topics, basically on any topics that you could name. You have probably browsed through one or participated in one. If you spend a lot of time on a certain topic, why not get paid for it at the same time? Paid forum posting is a way to earn money at home, doing what you enjoy.
You may wonder why web sites would pay for people to post. If you have searched for information on forums, then you would know that a forum without any new postings or helpful information wouldn't be of your interest. Your reaction is to leave the web site and go to another one. This is when webmasters need your help. They want to have people who will post often, make the forum appear active and interesting. They are searching for individuals who have knowledge on their particular topic and available to post and discuss the topic with others. With these individual participants, they can attract other enthusiasts to the forum. And in time, the site will be active and successful.
As an original poster, you may be required to stay on a forum for a certain period of time, and then move on to another one until the forum has reached its active level. Or you may stay around, and be known as a frequent and knowledgeable poster. Either way, you will be getting paid for what you enjoy doing, which you might have been doing for free anyway.