Campaigns are means of advertising using the services of people such as you and I as advertisers. Each campaign has a set rules guiding such campaign for as long as the campaign may last.
There are certain sections of the forum that you allowed to post according to the rules of the campaign you are in.
No,  you can be part of more than one campaign at a time.  There's no way possible to sell your advertising space to more than one company at a time.
Payment is made on a weekly basis according to your rank.  There are times when one does not get paid due flouting of rules in the campaign.