Down is the fluffy undercoating of ducks and geese, located just beneath their outer feathers. Down insulation breaths well, and drapes itself over your body reducing internal air spaces. It is also more comfortable over a wider range of temperatures. Although down has a high initial cost, because its useful life is measured in decades rather than years, it offers superb value in the long term. Down is graded for quality according to the number of cubic inches one ounce of down will displace when under a controlled pressure.
Many factors affect fill power.
Since few consumers are equipped to scientifically test down fill, your best protection is to buy established brand names from reputable retailers. Synthetic fills are non-allergenic, retain more of their insulative properties when wet, and have a lower initial cost than down. Synthetic insulations are extruded polymers, essentially plastic threads. Either type may be made with hollow threads. The hollow interior reduces weight, and traps air for insulation.