What is the difference between pillows designed for people who sleep on their back/side and pillows for people who sleep on their stomach?

I can't speak about: Any specific product designs.If you sleep on your back a puffy pillow which provides support but doesn’t put your neck at too much of an angle is recommended. If you sleep on your stomach, then no pillow or a thin pillow is best. When sleeping on your side a thick pillow can help to maintain your neck ; head in a neutral position.