As previously mentioned, this is the part you hold with your hands. It can be made of either rubber or leather. A grip can also vary in size to help you choose the right one that would suit your hand size and grip style.
However, a manufacturer cannot just design the grip without taking note of the regulations set by the United States Golf Association (USGA) and The Royal and Ancient Golf Club of Saint Andrews. Thus, the grip of a golf club needs to be round. It can have various holes or ridges, but they need to be minimal in size to meet the regulations. These grooves or holes are included to improve your overall grip on the golf club.