To put it simply — satin is a finish, not a fabric. Satin pillowcase or satin anything for that matter is basically a marketing ploy that has been going on for years. Synthetic satin — is what it really is. Synthetic satin (e.g. polyester) is, as the name suggests,  not a natural fabric. It may look shiny and has a nice and soft touch that natural silk has, but after hours of sleeping on it, your face and hair will feel the difference. Unlike silk, which is a 100% natural fiber, satin is petroleum-based. It is thinner, rougher, and it sweats, all of these characteristics make satin inferior to silk. Silk, on the other hand, has a charmeuse finish, which is, on the surface, similar to those of satin. However, in a closer look, a charmeuse finish is shinier, smoother, softer, and most importantly, it breathes better. You can easily tell a silk pillowcase from a satin pillowcase by spotting that faux, overly shiny reflective synthetic satin look.