What is the difference between the power cord grounding and equipment metal shell grounding

Power cord grounding and equipment metal casing grounding is to release the static electricity of the equipment, the power cord grounding is because our electricity is generally three-wire, a zero-one fire and a ground, the ground is generally true grounding, and equipment on the metal casing grounding is to not let the equipment static electricity is no longer gathered through the plug-in board to put a point, otherwise a magnetic metal casing may be electrified