A featherbed is made of feathers contained within a fabric shell that lies on top of a mattress as a mattress topper. The featherbed will normally have elastic straps or even have a fitted sheet on it so that it fits over a mattress and stays in place.
If you're in the market for king sheets, make sure they fit your mattress. King sheets should have deep pockets to accommodate larger king-size mattresses and prevent them from slipping. Also consider the sheet's thread count, which refers to the number of threads woven into one square inch of fabric.