What is the difference between down and feathers?

Down is the plumage forming the undercoating of waterfowl, consisting of tufts of light fluffy filaments (e.g. barbs) growing out of the cluster core without any quill shaft. Waterfowl down contains down cluster, half down, plumule down and damaged down. Waterfowls need down and feathers to keep their body temperature.

Feathers grow on the scarfskin, mostly on the necks, backs, and tails, which will help the waterfowls resist harsh weathers. In feather there is a curved inward quill, soft barbs on both sides, thousands of tiny barbs are crowded on the shaft.