They're what we refer to as forum cash. You get a point each time you make a post, although in certain areas posts are worth 0 points (such as the spam section). If you're extremely impatient, you can donate money to the forum, and you are rewarded with forum cash. Check out the rules: viewforum.php?f=73 Hopefully it will answer your questions.
viewtopic.php?f=73&t=16891 It's in that thread. It can be a way of saying thanks, or a way of deeming another person trustworthy. So you can give rep to people if they've done a smooth trade, or if they've written a great guide, etc