Duvets consist of a soft and protective outershell, usually cotton, polyester, or microfiber, and a inner fill, with the intention to provide comfort and warmth.   We break down the most common fill types here:
Down:  Down duvets refer to the fluffy cluster of down clusters or feathers, typically from the feathers of ducks and geese. Down products tend to offer the highest levels of warmth and are regarded for their ability to provide excellent insulation. Plus, it gives off that extra fluffy look. When choosing down products, be mindful that they have ethical production features like meeting the Responsible Down Standard (RDS), a common certification that confirms the down products derived from geese and ducks are raised in compliance with the principles and criteria of animal welfare. Most down is actually derived from animals who had been raised for food, while Live Plucking is considered harmful to animals.
Down Alternative: Down alternative products mimic the look and feel of down but are typically made of a synthetic material like polyester, microfiber or rayon or natural materials like wool or bamboo. They're typically considered a hypoallergenic option.
Wool:  Wool is another all-natural and sustainable design gaining market share, especially among hot sleepers, for its temperature regulating abilities, so you won't find yourself overheating or freezing. Wool is also moisture and odor resistant, and some organic options are available too. Wool is also naturally hypoallergenic and resistant to dust mites and allergens. A drawback to wool is that it does not offer the fluffy loft of standard duvets.
Silk:  Silk duvets and comforters definitely fall on the luxury end of the bedding spectrum, but provide many benefits including being naturally moisture wicking and thermoregulating, long-lasting (when cared for properly they can last for decades) hypoallergenic, breathable,
Note, silk tends to have a more demanding care and upkeep as it's typically not machine washable, compared to other materials.