Rubber and nylon are both soft and are less likely to damage whatever you are hammering, but rubber is even softer and bouncier than nylon. I would use rubber when pounding wooden pieces of furniture together, but nylon would work better for forming soft metal like jewelry. Other specialty hammers like brass and copper are non-sparking and non-magnetic for use around flammable gases and sensitive equipment. They continue up the hardness scale -- brass for softer applications and copper when you need more force. Finally, you have you traditional steel hammer that is usually made out of hardened steel and would really mess up that soft wood from earlier if you tried striking it directly.