REPutation points. When you see a post that you either feel strongly in agreement or find highly offensive, you can click on the little scales symbol at the bottom of the post to leave a positive or negative rating (REP) for that person.
Reputation. If you think a post or, more specifically, the poster is deserving of reputation... just click the scales under their user name and avatar to give feedback. The green blocks (just below their posts number) show how much rep they have accumulated. See your rep replies by clicking UserCP tab at top of page and scroll to bottom.
ETA: Dang, Crystalship beat me! Rep'd both of ya for the heck of it