it's basically a cushion that's filled with cherry pits you can heat in your microwave for about 3 minutes. It holds the warmth really long and gives it off to whichever bodypart you place it on (but it goes deeper than the heat from a normal warm water bottle).
I usually use that for big aches from the changes in season, my period, normal musclepain...
Oh. and when I'm freezing at night...
I love them and it was the first thing I and my sister bought when we went away to college. It's quite a basic necessity for us.
site for info on the first ones that were out, if you find it interesting, I know out here most stores have some in different brands, so you might find one via via where you're at. And the more popular ones come in teddy bears, but are less effective.