Down is a 3-dimensional spherical plume with many filaments radiating from it. Down is the light & fluffy coating clustered beneath feathers to protect birds from the elements.

Feathers have quills and are the rigid plumage found on the outside of a Goose or Duck.

All the tiny filaments and strands in down and feather lock together while forming tiny pockets where air is trapped. Down traps more air per ounce than any synthetic can. That interlocking of non-conducting air is what helps trap in warmth and keep the cold out.