This is to answer the question of what is "Rep"? "Rep" short for Reputation. Represents the reputation that you have in this community. The more you have the more people have liked what you posted whether it was a nice map, a great comment, or helpful information. Basically whatever you feel has been worthy of increasing another posters reputation.
If you look at the bottom left corner of a post under the member information you'll see a set of scales. Clicking that icon will make a window appear. You can add a comment as to why you agree with the post or how much you like it, and then click OK. That increases that posters "rep power" so that when they rep someone they in turn get more rep from that poster. It is a way to give some quantification to the quality of a particular posters posts.
If you click the User CP link at the top left corner of the page you'll see a Reputation section which lists the various comments that people have given you and a link to the post that earned you that rep.
Note: You can only give "Rep" to the same individual once per 20 people. So if you rep someone you won't be able to do so again until you have repped 20 others. Also you can only give out rep 5 times per 24 hour period. Assuming the default settings are being used.
Since the addition of some modifications to the system, you can now "like" individual posts. This will also add reputation, and should follow the existing reputation rules. Please let us know if you experience any problems.