That's the old rating system -- it was actually only per-user rather than per-post; you could be rated once by each other user, resulting in a rating score.
I was talking about back when there was something like a 'rating: + or -' link on everyone's post, it was based on the usefulness of that particular post and I think there was some variability to the amount of rating you got, perhaps 3 to 5 levels if my memory serves.
We now have "reputation" for users -- you can see underneath your username on the left that (as of the time of my response) you have a score of 16. I think they're planning on adding lots of extra ways of increasing this via different forms of participation, but currently it's affected by a per-post rating system; it's disabled for this forum, the lounge and other areas that are considered off-topic, but if you visit one of the technical forums you'll see each post asks "did you find this helpful?" (or something along those lines), and has a button to "vote up" the post. Some users also have the option to vote a post down if they thought it was unhelpful. All of these scores are added to your overall reputation.
Ok cool, so yeah that's sounds good and all, but I'd like to see a page where it shows the rankings, like a high scoreboard to make it more gamelike. I'd put it close to the 'Watched Topics' link.