The texture is rough since is has small scales similar to teeth, called Dermal Denticles.  Each species has a unique shaped denticle. They have a covering of dentine, a central pulp canal containing blood vessels and a single nerve. If you rub your hand along the body from front to back, there is little resistance and it feels smooth. However, when you rub your hand back to front, the skin feels like sand paper. The denticles play an important part in swimming efficiency. The water is channeled by the ‘skin teeth' and flows across the fins and around the body. The teeth also break up the interface between skin and water, reducing the friction between the two entities. The teeth skin also help protect the shark from injuries and several elements in the water. It's like a suit of armor for sharks.