Ply refers to the number of fibers used to create one thread. One-ply sheets have one fiber, while two-ply sheets have two fibers wrapped together to create a single thread. 

In its most basic form, microfiber is polyester woven extremely fine. Because microfiber doesn’t stain, they make great sheets for kids, if you’re eating in bed, and for those who sleep with their dog. It’s softer than polyester but less breathable than cotton making it good for cold weather.