What are down and feathers?

Down and feathers are found on virtually all birds. There is, however, a difference between down and feather.
The difference is mainly in their structure. As shown below, feather has a distinct quill and is uniform in its physical makeup. Although a cluster of down does have a quill, it is very small and may be undetectable. The structure of a down cluster is very undefined. The fibers are not connected or aligned with each other, as they are with feathers.