Force lightning is an offensive technique in the fictional Star Wars universe. Lighting extends from the fingers of the attacker, striking a target individual.
Force lighting is generally associated with the dark side of the force, and is usually only seen used by dark side practitioners such as Darth Sidius, and is sometimes reffered to as Sith lighting. Force lighting requires organic limbs and extremities, therefore Darth Vader, who has two prosthetic arms is unable to use this attack.
Lightning is not un-counterable, you can counter it as a dark or light users in many ways.
Lightning is where Fluk3 started this discussion, however, after chatting with him a bit, and reading his post again, I got the impression that is was less about lightning and more about removing a light force power to compensate the dark.
Lightning is a counter to light-forces and vice versa.