Fleece can be a confusing term as it can be applied to several fabrics. It refers to a soft and thick fabric with a deep pile.
Synthetic polyester fleece polarfleece or microfleece is often used as a water-resistant backing to pads. It can also be used as a top layer, where it works as a "stay dry" top, which wicks the wetness down into the pad. It can however feel hot and sweaty.
Cotton, bamboo, and hemp fleeces are natural fibre fabrics (hemp and bamboo fleeces are often a blend with cotton) often used to form the absorbent core of a pad. Occasionally the entire pad is made from the natural fleece.