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Rep power is how much you can change someone else's rep points by marking them as good or as bad. You get rep power by posting and by getting good rep points from other people.
Rep points are the number of rep points you have recieved from other people, and is represented in your posts by the little green or red bars. You can see the actual numeric value of your total rep points in your user cp under "latest reputation recieved". Each green bar represents about 100 rep points (from 1-100 points, you have 1 green bar, 101-200 you have 2 etc). I don't know the exact breakdown of rep-points to green bars, this is just a rough estimate mind you. If you have 0 or negative rep points, you will have no rep power and start getting red bars.
More details can be found here or here. Same text so take your pick.
I'd recomend that people consult FAQs before asking questions like this from now on, as the information is out there.
In the top, right hand corner of each post you will see a little icon with a check and a cross (check the rollover to be sure if you like). You click that and choose whether you approve or not, and can give a comment if you like.