Force lightning
Force lightning is a manifestation of the Dark Side of the Force used by some Sith as a weapon. Arcs of what appear to be electricity flow from the Sith's hand(s) into the target, causing pain, burns, and eventually death.
Appearances
Only two Sith Lords demonstrated the ability to use Force lightning in the Star Wars films. Chronologically, the first to use this attack was Count Dooku in Attack of the Clones. Darth Sidious later demonstrated this ability in Revenge of the Sith and again in Return of the Jedi.
Limitations
While debilitating to victims, Force lightning is apparently not an efficient way to kill. In fact, no victim of Force lightning died directly from the attack. Anakin Skywalker was incapacitated by Count Dooku's lightning for a few minutes, but he made a full recovery. Palpatine used his lightning to knock Mace Windu out of the window of the Supreme Chancellor's office, where Windu fell to his death.
Jedi of sufficient power and skill could repel Force lightning. Obi-Wan Kenobi was able to catch Count Dooku's lightning with his lightsabre blade, which was not damaged by the lightning. Similarly, Mace Windu was able to reflect Palpatine's lightning off his lightsabre back onto his attacker, causing Palpatine to be disfigured. Yoda was able to repel the lightning of both Count Dooku and Palpatine with his bare hands, although it was obviously more difficult against Palpatine.
Expanded Universe
Various fallen Jedi and Sith Lords have demonstrated the ability to use Force lightning in Star Wars novels and computer games, making it a far more common Sith ability in the Star Wars Expanded Universe.