Fill power is based on an objective test using one oz. of down in a tube with a weight placed on top of it. I posted a link to a site with the testing procedure on it a long time ago; might still be here somewhere. Here is what's posted on Wikipedia-http://en.wikipedia.org/wiki/Fill_power
There are links to a test lab at the bottom of the article-same one I linked to, I think.
The bottom line is that higher numbers mean more compressibility. How well an insulator works in a particular product will depend on construction. The idea is to create as much dead airspace between you and the outside air. A 650 jacket could be just as warm as an 800 jacket, just not a compressible and will weigh a bit more.