Reputation is "a way" everybody can see what members think of you and the answers you provide.
Gray rep - (not black) is from new members, if I'm correct below post count of 10 (correct me if I'm wrong)
Green rep - from these above 10 posts that agree/are happy with your respond. Their reputation power is different (according to member list top score goes to ancient dragon :) with currently 34 reputation point to give/take from any of us )
Orange rep - from these above 10 post that did not like your respond
I almost never give out negative rep in technical forums because I don't want to destroy anyone's reputation. I do give it out occasionally in geek's lounge because it doesn't affect overall rep points there. The rep for mods works the same as anyone else's, and the last time I heard it was 1/2 for negative rep.
The main reason I have such a large rep power is because of my huge post count. You get more rep for every 1,000 posts. And the more you post and participate in the technical boards the more frequently other members will give you good rep (assuming you post positive instead of negative things). My rep count doesn't mean I'm a better programmer than either Narue or Salem (which I don't think I am) ... it only means I participate in the forums more.