Down is the soft undercoating of Waterfowl (Geese and Ducks), consisting of clusters of light, fluffy filaments growing from a central quill point, without a quill shaft. A single down cluster is three dimensional and looks much like a dandelion pod. Down provides geese and ducks warmth and insulation, yet keeps the birds dry by allowing water and moisture to escape from the body of the bird, through the down, and into the air.
Down filled products are extremely resilient and insulate very well. In duvets, down is the best insulator per ounce over any other fill, natural or manmade. For pillows, down provides incredible re-lofting capabilities, yet is soft and comfortable.
There are many factors that affect the quality of down and its price.