A down comforter is a specially constructed comforter typically filled with down feathers from geese or ducks. Since down feathers tend to be light, warm and soft, a down comforter can create exceptional comfort without being too heavy. The quality of a down comforter is usually determined by its fill power, which measures how much space is occupied by the feathers. If the fill power is higher, it means the down comforter should feel fluffy. High-end constructions usually have a fill rating of 500 or higher while a more luxurious edition is rated at 700 or higher.
The stitching on a down comforter can affect its longevity and heating power. If it's quilted into lots of smaller squares, it's better at keeping sleepers warm but not too hot. This construction helps a down comforter last longer than one with sewn-through stitching. This style tends to have larger squares that keep the down evenly distributed throughout the comforter, which can cause it to clump up and sag over time.