Goose Down is simply the smallest, lightest feathers from a goose. Despite years of research and development, no-one has yet come up with a synthetic material that is as warm and lightweight as Goose Down. It's extremely compressible and loses none of it's warmth through being compressed. Russian Goose Down from geese brought up in a cold environment is widely regarded as being the warmest and best quality down. If treated properly, Goose Down can last for up to 20 years.
The two main uses for Goose Down are in sleeping bags and jackets. The compressability of the down mean that the garments or sleeping bags can be packed extremely small and return instantly to their normal size and shape without losing any of their warmth.
The warmth of down is down to the structure of the feathers, that consists of a central quill with thousands of separate fibres that interlock and repel each other at the same time. This produces millions of tiny air spaces that act as an insulating layer around you, keeping warm air in and cold air out.
The quality of down is judged by its elastic qualities, which are determined as "Fill power" or F.P. (volume to which an ounce of down returns to after being compressed). Fill Power tends to range from around 500 (nice and warm, good quality down) to 800 (roasting hot, top quality down).