Ok Rep is split into two sections. Rep Power and Rep Points

Reputation Power
Reputation Power is where it matters, this is the amount of REP you actually have. It is located right next to your name when ever you post anything. As you can see i have 21 Rep Power.
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You can gain Rep Power from Every 25 posts you get 1 Rep Power. You can also get 1 Rep Power for every 100 Rep Points.

Reputation Points

Reputation Points can olny be obtained if someone adds to your Rep. You can see how many Rep Points you have and how many you need on you account page. Near the bottom of the page under the "Reputation" tab.

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Giving/Taking Rep

If you want to add/take to someones Rep Points then you can easily do so by click on the little blue scale to the bottom left of there post, then selecting the amount you want to add/take and a reason why. To add Rep click "I agree with this Post" and to take Rep click "I Disagree with this post".

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Hopefully this will clear up any confussion with Rep, and if i have missed anything out tell me.