The term pinging is descriptive of the sound the problem makes. Technically, pinging is the collision of two seperate flame fronts within the cylinder. Normally, when fuel ignites, a flame front travels outwards from the sparkplug until all of the fuel in the cylinder is consumed. But, certain conditions can generate a second flame front started elsewhere in the cylinder. The second flame front is usually started by a hot carbon deposit. As for the damage, pinging doesn't really do any damage itself, but it can lead to detonation which melts down pistons and breaks spark plug electrodes filling your cylinder with shrapnel. Detonation is the explosive ignition of fuel. Detonation sounds like marbles in a can, but is usually fairly faint. You might not be able to hear it over your exhaust. If you experience detonation, you are using the wrong fuel, your spark plug is too hot, or your engine is improperly tuned.