100% cotton means all-cotton (except for Warm and Natural, which is 100% cotton needlepunched through a polyester scrim). 80/20 is typically 80% cotton/20% polyester. Polyester comes in different weights and/or thicknesses. A light weight is a thinner polyester; a heavier weight is a thicker polyester. "Fat batts" are polyester battings that have a lot of height and weight. In general, for hand quilting, you want to use a batting of thin or medium thickness no matter what the fiber content.