One of the strongest indicators of the quality of down is its fill power, which is a standardized measurement of loft that down provides. 

Fill power measures the volume of one ounce (28.35 grams) of down. It is measured by the following process: one ounce (28.35 grams) of down is placed in a graduated cylinder. A 68.4 gram weighted disk is placed on top of the down. Once the disk settles, the compressed volume, measured in cubic inches, is the fill power. So 700 fill power down has a volume of 700 cubic inches per ounce of down, 850 fill power down is 850 cubic inches per ounce of down, and 900 fill power down is 900 cubic inches per ounce of down. 

Not only is fill power a decent indicator of down quality, it becomes especially important in weight-conscious activities, such as mountaineering and backpacking. Since higher fill power down provides more volume of insulation per ounce, you need less total weight of down to insulate. For items with a lot of down insulation, such as sleeping bags and larger jackets, this could mean saving considerable weight, with the added benefit of better compression for a smaller packing size.

Fill weight is the physical weight of down used for a product. We measure the fill weight in ounces (or grams for the rest of the world). For instance, a jacket may have a fill weight of 9 ounces. That means there are 9 ounces of down inside - regardless of what fill power the down is. Knowing both the fill weight and fill power gives you a pretty solid baseline to compare products, because one of those numbers on its own doesn't give you enough information. A jacket with 9 ounces of 850 fill power down will be substantially warmer than a jacket with 9 ounces of 600 fill power down.