Fill Power measures how well the down lofts. The higher the loft the higher the fill power rating. Goose down fill power ratings in the outdoor industry generally vary between 600 and 850. Duck down fill power generally varies between 500 and 700.
Fill power affects the loft of the down and so crucially it measures the approximate warmth of a down item. For example a sleeping bag with 600g of 800 fill power down will be warmer than one with 600g of 600 fill power down. So a lower amount of a higher fill power down can be used to make a lighter bag of the same warmth.
The cluster ratio is the proportion of down to feathers. Processing the down removes almost all feathers, but it is impossible to get 100% down without any feather.
Fill weight is the total amount of down used in a complete item. This is generally measured in grams. Fill weight along with baffle construction is one of the largest influences on the overall warmth of the item. Very generally speaking the more down inside the item, the warmer it will be - although this is also largely influenced by fill power and cluster ratios.