From Wisegeek.com:
"To make Berber fleece, yarn is knitted into fabric, which is brushed with wire brushes to pull the material together and to fluff it up. The resulting material has a looped, soft pile, with large air pockets, which improve the insulating properties of the fabric. The pile is sheared to create an even length, and the Berber fleece is ready for use. Synthetic materials such as polyester are most frequently used to create Berber fleece, which tends to be strong, stretchy, and colorfast."