Here's how rep systems work in general:
Green rep: + to your rep total
red rep: - to rep total
grey rep: no rep given or taken, since this rep was given to you by a poster with a negative (red) rep level....red levllers cannot give rep of any form
Now, as to how hard a rep hit/gift is, depends on how the system is setup...some go by postcount, where for every x posts, your reps give y value. Others go by how much rep you currently have. IMO the second system is the better one, as it means others in the board, or at least a few in really high regard approve of your postings. The first system actually encourages postcount buffing, which is a waste of forum space.
Rep is not a bad thing, it just takes time to get used to it, and at first, every rep system is crap...it just takes time to mature and become useful as a quick gague to the quality of poster....if a guy has less than 20 posts and is already red, you can be sure that he's either a troll, or is very bad with words.