I heard under Ryan Eagle's tiedye shirt, there's just another fist.
PR is your Page Rank. Every site has an exact PR value, and they're constantly shifting.
It's based on things like domain age, reputation, the number of links you've got, the amount of regular updating and dynamic content, traffic, etc etc etc... Seriously, the list just goes on.
You can get a rough approximation of a site's PR with certain plugins for FireFox, like SearchStatus.