Down and feathers grow on the same bird, but they have a different task. Once you have felt the difference, you no longer forget it.
Feathers
Feathers sit mostly on the outside and on the wings. They protect against wind and moisture. They have a hard quill, are firmer and give structure. In pillows that can be useful.
Down
Down sits closer to the body. It has no hard quill and no fixed direction. It is light, round and built to hold air. That air provides insulation.
In a duvet
In a good duvet you find mostly down. Feathers may sometimes play a role for firmness, but the higher the share of down, the lighter and softer the whole usually feels.
Why it matters
The difference decides weight, suppleness and comfort. Down belongs in what you pull over you at night. Feathers have their place where more support is needed.
How you feel the difference
Take a feather and a tuft of down in your hand. The feather has a direction and a central quill. Down has neither. It is softer, rounder and airier. That difference explains why both materials feel different in bed.