You get points for every post you make. I don't know if it is explained anywhere, but I have noticed you get 5 for a post, 20 if someone hits reply or quote to your post and then additional points for more replies and quotes that I haven't figured out yet. It kind of gives you an (admitedly meaningless) incentive to make thoughtful posts that others will want to reply to - and keep threads from dying.
Essentially, take your points and divide by posts to find out how cool you are.... Bob, why do you have about the same amount of posts as me but so few points [:P] What are you - a thread killer?
Oh, and I recently figured out how you get the green bars under your avatar. This is based on number of posts - and if you hover over the bar it tells you that you are in the top number of posters (if you ahve any green bar). 1 bar is top 500 (around 110 posts I think), 2 bars is top 250, (about 225 posts), 3 bars is top 150 (about 300 posts), etc.