Electric spark we call lightning than jumps between differently charged parts of clouds. In our body electrical signals travel along nerves carrying information from and to the brain. Electrical impulses from the brain tell our muscles to move, our heart to beat, lungs to breathe. Our whole body is controlled by our brain this way. The information is sent from every system than back to brain.
Electric shock is passage of electric current through the body.
It destroys body's own electrical signals normally travelling along the nerves to regulate whole body. As a result heart stops beating, our body stops breathing and all systems functioning. Electric current also causes burns of skin and muscles.