To a lot of companies satin is a vague term because it is a man-made material. Technically it can be made of anything that feels smooth to the touch. I've used "satin" bonnets and things that were made of polyester to cover my hair at night when I sleep and they're all right. They prevent breakage just as they should, but you can really tell the difference between satin and silk when you start sleeping with 100% silk scarves. My hair feels a lot smoother, softer, and looks shinier, and companies can't lie to you and say this is a 100% silk scarf and give you polyester because silk is a natural fiber. Silk is more expensive than satin/polyester (depending on where you go) but, in my opinion, it's so much better.
P.S. Polyester is essentially plastic fibers. If you are looking for something more natural for your hair you might want to look into silk.