Down is the soft under plumage (a layer of insulation underneath feathers) that geese, ducks and other waterfowl have to keep them warm and dry. Feathers have hard quills, down does not. A down "cluster" is soft and fluffy and will have many filaments growing in all directions. A down cluster looks like this:
Fill power or "loft" is how the quality of various goose down is defined. The larger the down cluster, the higher the fill power rating. Fill power is the number of cubic inches one ounce of down will fill under specific laboratory testing conditions. Fill power can range from 300 cubic inches per ounce, to 800 and above for the finest quality large cluster goose down.
Goose down and duck down are basically very similar. However, because geese are larger than ducks, the goose down clusters will be larger than duck down, resulting in better insulation for down comforters and fluffier filling for down pillows. Larger waterfowl = larger down clusters.
Goose down can vary in color, from white to dark gray depending on the age of the bird and the season in which the down was harvested. The color of goose down does not affect its ability to insulate - in other words, a dark gray 650 fill power goose down will be just as fluffy and insulate just as well as a white 650 fill power goose down. Aesthetically speaking, white goose down is generally more preferable to gray goose down since a gray down can show through a fine white down proof ticking, making the comforter appear "dingy".
Ounce for ounce, goose down is the finest lightweight insulator known, natural or synthetic. It's unique three dimensional structure creates thousands of tiny air pockets. These air pockets, trapped by the down fibers, provide the superior insulating ability that goose down is known for. Quality goose down is also extremely resilient - the ability to be compressed then spring back to it's original shape with all of those wonderfully warm air pockets. No other material is as resilient as a high quality large cluster down.
Goose down is also extremely breathable compared to other materials. It allows water vapor to pass through, wicking it away through the filaments while retaining the warmth in the air pockets.