It's mostly a popularity poll. And it has quite a convoluted scheme:
You get a rep point for a certain number of postings.
You get rep points according to the person who reps you (if someone with 10 rep power reps you, you get 10 points, if someone with 20 rep power reps you you get 20 points).
At certain points milestones you get another rep power point (never have figured out exactly how many points it takes to achieve the next power point).
Clear as mud, eh?